Buffalo and Fisher inbred rats and their F1–F4 hybrids were exposed to heat, 96 °F and 30% humidity, at 35 days of age. Fisher rats which are of lighter weight survive significantly longer than Buffalo rats which are heavier. Body weight was significantly depressed in all hybrids. Only in F2 hybrids was heat tolerance similar to that of the Fisher inbreds. In all other hybrids, it was lower. No heterosis was found under the conditions us