This investigation was designed to determine if a correlation exists between a change in the response of the hypothalamo-hypophysial axis to castration and the onset of refractoriness in male hamsters exposed to short days (in this case LD 6:18). The castration response, here defined as a significant increase in serum gonadotropins over levels in intact males, in males exposed to short days for 2 weeks was no different from that observed in long day males. On weeks 4 and 6 the response was greatly attenuated, and on weeks 8 and 10 no castration response was observed. Refractoriness was first observed in a few animals exposed to LD 6:18 for 8 weeks, and in increasing numbers of animals on weeks 10, 12, 13 and 14 of short-day treatment. Thus, short day exposure results in a simultaneous loss of response of the hypothalamo-hypophysial axis to castration and initiation of refractoriness.