Lord Agni in Agni Purana has discussed various Mantras. Knowledge of these mantras grants enjoyment, comforts in this life and salvation in the next. It has been said in the Purana that the Mala-Mantras consist of more than twenty letters, the Vijas in order to be classed as Arvak-Mantras must consist of more than ten letters. The Arvak-Mantras constantly repeated, bear fruits at their old age, i.e., they grant salvation to their reciter at the lapse of the full period for which they are enjoined to be repeated. The Vija-Mantras which generally consist of less than ten letters, fructify at the middle of their enjoined time, i.e., at their youth. The Mantras containing of more than five and less than ten letters, may bear fruit at any time of their ordained period of repetition.

Agni Purana has mentioned that the Mantras are usually reckoned as belonging to masculine, feminine, or to the neuter sex. The feminine Mantra invariably end with the term Swaha, the name of the wife of the Fire-god, while those that are of the neuter sex, end with term Namas (obeisance), the rest being held as masculine. The Mantras of the male sex should be used in charms, or in those mystic practices which tend to distract the minds of one`s enemies. The feminine Mantras should be made use of in acts of little moment, or in those which are undertaken with the sole object of bringing sound health on them on whose behalf they are practised. The Mantras which are reckoned as belonging to the female sex should be used in all other sorts of charms. The Mantras are again grouped under the two broader sub-divisions of Soumya, (soothing), and Agneya (fiery). The Vijas known as the Tara should be appended to both the Soumya, and the Agneya Mantras. The Agneya Mantras usually end with a Tara-Vija. The Soumya or the Agneya Mantras should be used in an act which is of a nature quite in conformity with the meaning of either of them. An Agneya Mantra may be converted into a Soumya one, by adding the term Fut (obeisance) to its end.

According to Agni Purana, a Mantra which is asleep, or has been just aroused out of its state of slumber, does not bear fruit, though ardently repeated in rapid succession and for ages and ages together. The sleeping state of a Mantra should be ascertained from the rapid and fluttered emission of breath through both the nostrils, while its waking should be judged from the calm and uniform passing of the wind through the left. An Agneya-Mantra may be converted into one of the Soumya class, by repeating its component letters in an inverse order. The state of waking of both these Mantras should be carefully ascertained, and letters or Vijas which are presided over by baneful planets and asterisms, should be scrupulously excluded therefore. In acts of charm, practised for the acquisition of a kingdom, or for the benefit of an individual, the vowel letters which are inimical to the component letters of his name, should be first arranged in due order.

The Mantras are to be ascertained from computations made on the table on which the Gopal-Mantras are calculated, or by means of a Nakshatra Chakra. The letters of the alphabet from A to AS excepting the two Ris and the two Lis, should be arranged under the different asterisms from Ashvmi to Rohini, or the different chambers of the former diagram should be marked as the Siddha, Sadhya, the Susiddha and the Ari and the character of each Mantra in relation to the name of the individual to whom it should be imparted, should be calculated there from. A man, by simply repeating a Siddha-Mantra, attains salvation, while a Sadhya-Mantra requires the performance of the rites of Homa and worship, to be at all fruitful. A mere contemplation of a Susiddha-Mantra is enough for the salvation of its votary. A repetition of a hostile Mantra is sure to lead its votaries to the grave. A Mantra, full of forbidden or baneful letters of the alphabet, should be carefully avoided.

Agni Purana states that a man, having been duly initiated into the mysteries of a Tantric worship, and having been duly blest with the rite of final Ablution (Abhisheka) by his religious preceptor, should repeat the Mantras, obtained from him. A Brahmana who is scrupulously truthful, pure, and intelligent, possessed of god-like fortitude, given to Japa and meditation, fully conversant with the books of Scripture, practises penances, and is capable of elevating or chastening his disciple, should be acknowledged as a Guru. A disciple, on the other hand, should live on a purely vegetable or Havishya diet, cultivate liberal sentiments, never allow his mind to be ruffled by any consideration of the world, practise self-control and live the highest intellectual life possible, and should be a man always eager to learn fresh spiritual laws, heartily devoted to the service of his Guru, whom he should help with money, if necessary. A Guru, having imparted a Mantra to his disciple, should repeat it ten thousand times for its perfect fruition.

A Mantra heard at random, or obtained by force or stratagem from a Guru, or any Mantra found written on a leaf, or imparted in the form of a gatha, should be looked upon as absolutely barren. The man, who had nearly perfected a Mantra with rites of Homa and worship in a previous existence, is sure to work out his salvation in his next or succeeding birth, with a little labour upon the same.

It has been said that a Mantra consisting of a single Mantra, bears fruit after being repeated a million times A Mantra consisting of a good many number of letters, should not be repeated so many times, while a Mantra consisting of letters, numbering twice or three as much as its Vija-Mantras, should be repeated in the way of a Rosary-Mantra. A Mantra should be repeated hundred and eight, or a thousand, times, where no definite number of repetition would be mentioned and in all cases, libations of clarified butter numbering a tenth part of the number of Mantras repeated, should be poured on the sacrificial fire. Libations should consist of clarified butter alone, where no particular substance would be enjoined in that behalf. A man, incapable of performing the rite of necessary Homa, should further repeat a tenth part of the number of the Mula-Mantras, originally recited. The presiding deity of each Mantra, duly evoked, is pleased with the final ceremony of Homa, worship and meditation, and grants the wished-for boon to the votary.

Similarly it has also been stated in the Agni Purana by the Fire-God that a Mantra repeated in a small voice, should be deemed as ten times more meritorious than the one repeated in a loud tone, whereas the one repeated in a voice scarcely audible, but which is only judged from the constant movements of the tongue, is hundred times more efficacious than the latter, while a mental repetition of the Mantra, should be deemed as a thousand times more fruitful than that.

According to the Agni Purana for the absolute fruition of the Mantra the votary should live on a Havishya diet, or take wheat-cakes soaked in milk, on the day of its repetition. The regent of the Mantra should be worshipped under the auspices of the eighth or the fourteenth day of the moon`s wane, or on the occasion of a solar or a lunar eclipse.