Body temperatures were measured with a thermistor inserted through the rectum in intact and hypophysectomized male C3H mice, kept under standardized conditions of environmental temperature and lighting. At 1 month after hypophysectomy, temperature measurements were made on all the mice at 4-hour intervals during a 4-day period. At about 5 months after operation, measurements were made once, on separate subgroups of mice, at a 12-hour interval; this experiment was repeated 3 days later. At 1 month after hypophysectomy, the mean temperature was lower, the amplitude of daily change was smaller, and the period was slightly shorter, compared to intact controls, but there remained a significant difference between night and day mean temperatures. At about 5 months after operation, daily variations in the group mean were no longer of significant amplitude at the two times when they were measured, compared to those in intact mice.