Breeding stocks of mice of strain A2G have been studied at environmental temperatures of −3° and 21° C. The mean age of opening of the vagina was 33 days at −3° C, 26 days at 21° C. The mean body weight at opening was 13 g in both temperatures. The vaginal smear of typical oestrus appeared at a mean age of 61 days at −3° C, 38 days at 21° C; it was preceded by variable numbers of anomalous smears containing squamous cells. The mean length of the oestrous cycle was 8·5 days at −3° C, 4·8 days at 21° C. Females transferred from 21° to −3° C had longer cycles at first, but tended to return to normal after some months. The interval between parturitions had two modes, at about 3 and 6 weeks respectively: most intervals were around 6 weeks at −3° C, 3 weeks at 21° C. There was evidence of a negative correlation between the numbers weaned in successive parturitions, when the interval between parturitions was near the minimum. The slowing of the reproductive cycle at −3° C may be attributed to the prior demands of catabolism; but this does not account for the recovery of the mice transferred from warm to cold.