The development of the temperature difference between the rectum and testis has been studied in 20-, 25-, 30- and 35-day-old as well as adult rats. Temperatures were recorded with thermocouple needles without anaesthesia. The temperature difference was approximately 3·5° C in the adult and 35-day-old animals. The difference was about 2° C in the 25- and 30-day-old rats and about 1° C in the youngest ones. The three younger groups (20, 25 and 30 days) had significantly higher intratesticular temperatures than the adult rats. In the young rats variations in body temperature between 35·6° C and 38·4° C did not significantly affect the rectum-testis temperature difference. In the adult rats, the difference began to increase when the rectal temperature rose above 37·6° C. The relationship of developing temperature difference to the onset of endocrine puberty, functional maturity of the seminiferous tubules and development of the scrotal sac is discussed.