The effect of climatic conditions during summer on different heat tolerance indicators was determined in Friesian and Jersey cows which were kept in open camps with no protection against solar radiation. A complete diet (15.0% CP and 10.8 MJ ME/kg) was fed daily on an ad libitum basis in fence-line feeding troughs. Heart rates, respiration rates and rectal temperatures of five primiparous Friesian and five primiparous Jersey cows, which were accustomed to handling, were measured at two-hourly intervals from 07:00 to 19:00 over 15 days during which high (≥27.1 °C ) maximum temperatures were expected. Rectal temperatures of Friesian cows were higher (P ≤ 0.05) than those of the Jersey cows from 11:00 to 19:00, with the greatest difference (0.55 °C) recorded at 15:00. The respiration rate of Friesian cows was higher (P ≤0.05) than that of Jersey cows at 15:00, 17:00 and 19:00, with the greatest difference (15.4 inhalations per minute) recorded at 15:00. The heart rate of the Friesian group was higher (P ≤ 0.05) than that of the Jersey cows at 15:00 and 17:00. Heart rate was not influenced by increasing ambient temperatures to the same extent as rectal temperature and respiration rate.  Results suggest that Jersey cows display a higher heat tolerance than Friesian cows, and that Jerseys should be more widely used in the warmer regions of South Africa.