Male rats (120 g) either were subjected to a 12-wk physical training program (T rats) or were sedentary controls (C rats). Subsequently the rats were killed at rest or after a 45- or 90-min forced swim. At rest, T rats had higher liver and muscle glycogen concentrations but lower plasma insulin. During exercise, blood glucose increased 60% in T rats but decreased 20% in C rats. Plasma glucagon and insulin concentrations did not change in T rats but plasma glucagon increased and insulin decreased markedly in C rats. Plasma epinephrine (90 min: range, 0.78-2.96 ng-ml-1, (T) vs. 4.42-15.67 (C)) and norepinephrine (90 min: 0.70-2.22 (T) vs. 2.50-6.10 (C)) were lower in T than in C rats. Hepatic glycogen decreased substantially and, as with muscle glycogen, the decrease was parallel in T and C rats. The plasma concentrations of free fatty acids were higher but lactate and alanine lower in T than in C rats. In trained rats the hormonal response to exercise is blunted partly due to higher glucose concentrations. In these rats adipose tissue sensitivity to catecholamines is increased, and changes in glucagon and insulin concentrations are not necessary for increased lipolysis and hepatic glycogen depletion during exercise.