1. The temperature of the surface of the scrota of three 4-month-old Ayrshire calves has been measured in environments of 15, 20, 25, 30, 35 and 40° C. at a low humidity of 17 mg./l. absolute humidity and in environments of 30, 35 and 40° C. at a high humidity of 7 mg./l. saturation deficit. Five replicate experiments were performed at each environment on each animal and the animals were exposed to each environment for 6 hr.; measurements of scrotal temperature were made once every 5 min.

2. The scrotal surface temperature increased with increasing environmental temperature ranging from 32° C. at 15° C. to 39° C. at 40° C. The rates of increase in scrotal surface temperature with environmental temperature were curvilinear for two of the animals and rectilinear for the other. For the two whose rates of increase were curvilinear the rate of increase was constant at 0·2° C./° C. environmental temperature in the range 15–25° C.