The release of LH, FSH, and prolactin has been investigated in intact male rats, in estrogen-treated male rats, and in castrated male rats. The injection of 2 µl of 0.15 m NaCl into the third ventricle to cause ventricular expansion in castrated animals stimulated markedly the release of LH. FSH release was also stimulated but only slightly. Prolactin release was inhibited. Intact rats or estrogen-treated rats responded not at all or to a much lesser extent than did the castrated rats. Serotonin creatinine sulfate, given intraventricularly, stimulated the release of LH in intact rats but suppressed the release of LH in castrated rats that occurred following ventricular expansion in these animals. N-acetyl-serotonin generally had no effect on LH release in intact rats, but, in an occasional animal, this indole markedly stimulated the release of LH. In castrated animals, N-acetyl-serotonin and melatonin suppressed LH release. These indoles appeared to have little effect on FSH release in these experiments. Prolactin release was stimulated by N-acetyl-serotonin.