The effects of MSH on spontaneous and induced ovulation as well as on serum LH levels were studied. Rats which received progesterone on D1 and D3 were injected i.p. with 30 μg of α-MSH or saline solution and examined for ovulation on the following day. The injection of α-MSH induced ovulation in 90% of the animals; saline solution, on the contrary, induced an ovulatory response only in 10% of the cases. Rats injected with β-MSH yielded similar results to those injected with saline solution, indicating a possible specific α-MSH action. The replacement of the second progesterone injection by α-MSH failed to increase ovulation indicating that α-MSH synergizes or alters progesterone action. It would seem that α-MSH stimulates ovulation-potentiating progesterone effects on the CNS. LH effectively induced ovulation in rats where spontaneous ovulation was blocked by an injection of progesterone on D1. The i.p. injection of 30 μg α-MSH lowered the amount of LH necessary to induce ovulation. Rats which received α-MSH and LRH showed a marked increase in serum LH levels compared with rats where α-MSH had been replaced by saline solution. Collectively the information provided in this paper is consistent with the view that the release of MSH observed in the morning of PE would signal the discharge of LH from the pituitary gland occurring in the afte