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Experiments were conducted to evaluate the heat tolerance of the following breeds: Hereford (H), Brahman (B), H × B, H × Boran (H × Bo), and H × Tuli (H × T). Heat tolerance was evaluated in a climatically controlled room (Exp. 1) and under summer environmental conditions (Exp. 2) by comparing rectal temperatures (RT), respiration rates (RR), and sweating rates (SW). In Exp. 1, under extremely hot conditions (mean temperature-humidity index [THI] > 90), purebred B had significantly (P < .05) lower RT and RR than other genotypes, which may be indicative of greater surface area per mass to dissipate heat and a lower metabolic rate than other genotypes. Boran and Tuli crosses had RT (39.5°C) that were intermediate to those of B (39.0°C) and H × B (40.0°C). The H genotype had the greatest RT at 40.3°C. Among the breeds, trends in RR were similar to RR observed at THI < 77; B had the lowest RR, and …