Rats fed meals of high- and low- carbohydrate diets (2 hours daily) were either forced to swim at 38 - 40° C for 1-1 /2 hours daily for 15 days .or kept in a 7° C cold room.for approximately 4 weeks. On the low-carbohydrate diet, liver glutamic pyruvic transaminase (GPT) and glucose-6-phosphatase (G-6-Pase) were elevated in both swimmers and cold-exposed rats. In high-carbohydrate-fed swimming and cold-exposed rats, liver GPT was elevated, but G-6-Pase rose only in the cold-exposed rats. Peak liver glycogen in the high-carbohydrate-fed swimmers was depressed, while in high-carbohydrate-fed., cold-exposed rats it reached higher levels following meals. High-fat diets did not adversely affect swimming capacity of rats. It is suggested that cyclic events in the glycolytic components of livers of rats fed carbohydrate meals are altered in different ways and thus preclude using cold as a model for exercise. When high-fat diets are fed, these cycles are minimized and the responses to the two stresses are similar.