To investigate whether exercise in coxsackie B3 virus infection is detrimental to the myocardium, Balb/c mice were inoculated with the virus and exercised to exhaustion on a motor-driven treadmill up to 48 hours after the inoculation. This infection evokes myocarditis. The inflammatory and necrotic lesions in the ventricular myocardium 7 days after the inoculation covered 4.32% of the tissue section area in the nonexercised group. Exercise at 0 hours did not affect this myocardial damage (4.77%), whereas exercise at 48 hours after the inoculation increased the lesion to 7.85% (p less than 0.05). Lethality was not influenced by exercise. The response pattern of myocardial lymphocyte subpopulations was studied with an immune histochemical staining technique. The number of T cytotoxic, T suppressor cells increased threefold (p less than 0.01), and the T cytotoxic, suppressor/T helper cell ratio increased twofold (p less than 0.01) with exercise at 48 hours but was unchanged with exercise at 0 hours. The number of class II expressing cells decreased with exercise at 48 hours (p less than 0.05) and was negatively correlated (p less than 0.01) with the size of the inflammatory reaction. The development of myocardial inflammatory and necrotic lesions seems to be dependent on the presence and cooperation of class II expressing cells and T killer cells. Furthermore, failure to restrict physical activity in the acute phase of this infection may well contribute to the progression of the disease.