The extreme regularity of the oestrous cycle of the hamster was first noticed by Deanesly (1938). In our colony, 4-day cycles are maintained for at least a month, although no attempt is made to regulate the light-dark regimen. This suggests that the hamster cycle is not affected by light. In contrast, rats subjected to continuous illumination soon stop ovulating and develop persistent vaginal cornification; this syndrome has been termed 'constant oestrus' (Browman, 1937; Hemmingsen & Krarup, 1937; Fiske, 1941; Dempsey & Searles, 1943). The following experiment was therefore undertaken to compare the effects of continuous light on the oestrous cycles of the hamster and the rat. Sixteen female golden hamsters (Cricetus auratus) were placed in a constantly illuminated room after seven consecutive 4-day oestrous cycles. The animals were placed in groups of four in wire cages; the average illumination, determined by a photometer, was 40 ft.-candles per sq. ft. The