The sexual activity of groups of Merino, Dorset Horn and Border Leicester rams, from which weekly semen collections were made by artificial vagina, was studied at controlled temperatures from 27 to 43° C. Only Merino rams maintained sexual activity at the highest temperature (43° C). Dorset Horns recovered activity after the temperature was lowered but Border Leicesters did not. Semen motility was poor at high temperatures in all but two rams, both Dorset Horns. The Merino rams had shorter reaction times than the other breeds. There was no significant correlation between reaction time and number of ejaculates. The rectal temperatures of the Dorset Horn rams were 0·3 to 0·4° C lower than those of the Border Leicesters; Merinos were intermediate.
At a given ambient temperature, rams had lower rectal temperatures after severe heating than before, apparently owing to increased thermoregulatory ability. Mounting and ejaculation resulted in virtually no rise in rectal temperature.