Summary. Rectal temperatures of 934 Jersey and Australian Illawarra Shorthorn dairy cows were recorded during summer months immediately before artificial insemination. They ranged from 98·8°F to 105·0°F (mean 101·6 ± 0·9°F) and were related directly to stage of oestrus, supporting an earlier report of a thermal response at oestrus. Thus, the temperature of the cow's genitalia may be high during oestrus; and the sperms may be subjected to such environment for several hours after insemination. In addition, the data support the hypothesis that the normal diurnal rhythm of body temperature is restored some hours before ovulation occurs. Under such conditions, high rectal temperatures at the time of insemination were not inimical to the subsequent fertilization process. Indeed, in one group of cows, inseminated at a late stage of oestrus, fertility was better (P <0·05) in cows with elevated temperatures than in those with low values.