The effect of forced exercise on the development of coxsackievirus B3 myocarditis in inbred C3H/HeJ mice was studied. Four groups of mice (30 per group) were formed: infected-exercised (Group I); infected-unexercised (Group II); uninfected-exercised (Group III); and uninfected-unexercised (Group IV). Infected mice were inoculated intraperitoneally with 1.0 x 10(2.1) TCID50 coxsackievirus B3. Exercised animals were swum daily for 60 minutes on days 1-9. Myocardial viral titers were acutely elevated on day 3 of infection and were augmented significantly by exercise on days 6 and 9. Exercise increased the overall mortality from 0-10% to 20-40%; significantly increased heart: body weight ratios on days 6, 9 and 13; and increased the extent of myocardial fiber necrosis. We have reproduced the acceleration of CB3 myocarditis by exercise in the inbred C3H model.