Apollodorus tells how the gift first came to him. Melampus, son of Amythaon and Idomene and brother of Bias, lived in the country, and before his house stood an oak that held a lair of snakes. His servants killed the grown serpents, but he gathered wood, burnt the reptiles, and reared their young; and when they were full grown they stood at each of his shoulders as he slept and purged his ears with their tongues. He started up in a fright and found that he understood the voices of the birds flying overhead, and from what they told him he foretold to men what should come to pass. He learned besides the art of taking auspices, and having met Apollo at the Alpheus he was ever after an excellent soothsayer.2Pseudo-Apollodorus — Bibliotheca 1.9.11

His brother Bias wooed Pero, daughter of Neleus, whom many sought; but Neleus would give her to none save the man who drove off the cattle of Phylacus from Phylace, kine guarded by a dog no man or beast could approach. Melampus undertook the theft for his brother's sake and foretold his own fate exactly: that he would be caught and held in bondage a year before the cattle were his. So it fell out, and in his cell he overheard the worms in the roof-beam reckoning how little of the beam was left ungnawed; he had himself moved to another cell, and the first fell in. Marvelling, Phylacus released him and asked how his childless son Iphiclus might beget. From a vulture Melampus learned that a gelding-knife once left bloody beside the boy, then driven in fright into the sacred oak and swallowed by the bark, was the cause; found, its rust scraped off and drunk ten days, it would heal him. So Podarces was born, the cattle went to Pylos, and Pero was won for Bias.3Pseudo-Apollodorus — Bibliotheca 1.9.12

Homer tells the same affliction more darkly and carries the house forward. A seer of his far-off line, says the bard, was sprung from Melampus, who of old dwelt in flock-rich Pylos until he fled his country and great-hearted Neleus, who for a full year kept much wealth from him by force. Melampus lay bound in bitter bonds in the halls of Phylacus, suffering grievous pains because of the daughter of Neleus and the blindness of heart an Erinys had laid upon him; yet he escaped his fate, drove the deep-lowing kine from Phylace to Pylos, avenged the cruel deed on godlike Neleus, and brought the maiden home to be his own brother's wife. For himself he went to horse-pasturing Argos to rule over many Argives, wedded a wife, and begot Antiphates and Mantius, from whom the seer-line of Amphiaraus and Polypheides descends.4Homer — Odyssey 15.225–256

His greatest cure won him a kingdom. When the daughters of Proetus went mad and roamed the Argive land, Melampus, being a seer and the first to devise cure by drugs and purifications, promised healing for a third of the sovereignty; Proetus refused, the plague spread to all the women, and only when the evil reached its height did the king consent, on the harder terms that Bias too should have a share. Melampus then chased the women from the mountains to Sicyon with shouts and a frenzied dance; the eldest daughter, Iphinoe, died in the pursuit, but the rest were purified and recovered their wits, and Proetus gave them in marriage to the two brothers.5Pseudo-Apollodorus — Bibliotheca 2.2.2

Pausanias keeps the bargain's issue and the seer's cult. He alone notes that the Argives were divided into three kingdoms: in the reign of Anaxagoras the women were smitten with madness, and Melampus cured them on condition that he and his brother Bias have a share of the kingdom equal to the king's, after which a line of Bias's descendants, Neleids on their mother's side, long held the throne. At Aegosthena, Pausanias adds, there is a sanctuary of Melampus, son of Amythaon, with a small figure carved on a slab, where they sacrifice and hold a yearly festival.6Pausanias — Description of Greece 2.18.4; 1.44.5