Mitochondrial preparations from stalk median eminence of female rats were shown to contain an enzymatic system which yielded MSH-RF upon incubation with oxytocin. These enzymes were found present in the hypothalamus of rats at proestrus and estrus and absent in the other stages of the cycle. The MSH contained in plasma was also determined during the estrous cycle in the rat. Serum MSH was released on the morning of proestrus, reached a maximum value at 10.00 a.m., and remained high until 4.00 p.m. Thereafter serum MSH decreased to undetectable levels in the other stages of the cycle. MSH was released into the serum about 6 h before the proestrous peak of the gonadotrophin. A correlation was demonstrated between the hypothalamic enzymes responsible for the formation of both MSH-R-IF and MSH-RF and the MSH released into the serum.