27 sexually-inexperienced male rats were maintained under artificial lighting conditions. The living cages were illuminated for 12 hrs. and darkened for a similar period. All animals were observed in mating tests with receptive females. Males tested during the dark phase of their diurnal cycle were much more active sexually than others tested in the lighted half of the period. When tests were shifted from the light to the dark phase, mating reactions increased promptly. In response to the reverse change sexual activity was reduced. Lowering of responsiveness which accompanied a shift from dark to light phase of the cycle was compensated in part by the effects of experience gained during previous tests conducted in the dark phase. It is concluded that the male rat''s tendency to engage in sexual behavior is greater at night than in the day, and that the difference is probably due to the higher metabolic rate which characterizes the early nocturnal portion of the diurnal cycle.