The prince, Proetus' son, suffered from a strange malady and the king offered a reward for anybody that could heal him. Melampus, a local seer, killed an ox and talked to the vultures that came to eat the corpse. They said that the last time they had had such a feast was when the king made a sacrifice. They told Melampus that the prince had been frightened of the big, bloody knife and the king tossed it aside to calm the child. It had hit a tree and injured a hamadryad, who cursed the prince with the sickness. The hamadryad told Melampus that he boy would be healed if the knife was taken out of the trunk of the tree and boiled, then the rusty water that resulted dranken by the prince. Melampus followed her directions and demanded two thirds of the kingdom for himself, and one third for his brother, Bias. The king agreed.
Alternate:
When the women of Argos were driven mad by Dionysus, in the reign of Proetus, Melampus was brought in to cure them, but demanded a third of the kingdom as payment. The king refused, but the women became wilder then ever, and he was forced to seek out Melampus again, who this time demanded both a third for himself and another third for his brother Bias.
Sometimes the madness of the Argive women, and their cure by Bias and Melampus in exchange for a third of the kingdom each, is said to have occurred during the reign of Proetus. However, it is also said to have taken place during the reign of Megapenthes' grandson Anaxagoras, and since it was Argos that was ruled over by three kings, the former version may be a simple mistake.
Proetus, possibly a different Proetus from the above, was the father of Maera.