Subcutaneous Silastic implants were designed to release quantities of testosterone approximating that produced by the rat testis and appropriate testosterone treatment was found to produce a physiologic inhibition of both LH and FSH secretion in orchidectomized rats. In rats bearing such testosterone implants, intravenous injection of an ovine anti-testosterone serum (0.3 cc) was sufficient to completely abolish the inhibitory effects of the testosterone implant for a period of 3 days as judged by the development of typical post-castration increases in serum LH and FSH. When this dose of antiserum was administered to intact adult male rats, the increases in serum LH on days 1, 2, and 3 post-injection were approximately 19%, 15%, and 11% of those observed in untreated castrates. Increases in serum FSH in antiserum injected rats followed a pattern similar to that for serum LH but the response was slightly greater (19%, 23%, and 22% of the castrate response) and there was no indication of a decreasing effect during the three days following injection. In these experiments, passive immunization against testosterone in intact male rats produced a clearcut stimulation of both LH and FSH release, but produced no data to support the suggestion that the intact testis can secrete nonsteroidal compounds capable of producing a differential inhibition of FSH secretion.