Ten long distance runners were enrolled in a 6-hour competitive race. Immediately after the race technetium-99m-albumin gated blood pool scans were performed and indium-111 antimyosin was injected. Forty-eight hours later antimyosin scans were obtained and control gated blood pool scans were performed. Left ventricular ejection fraction was higher after the race (65 +/- 5 vs 60 +/- 7%, p less than 0.01) due to a decrease in end-systolic counts. Right ventricular ejection fraction was lower after the race (42 +/- 7 vs 54 +/- 12%, p = 0.03) due to an increase in both end-diastolic and end-systolic counts. A longer systolic period was observed after the race (53 +/- 5% of the RR interval vs 39 +/- 3%, p = 0.005). No significant differences were observed in peak filling or peak emptying rates after the race. An increase in pulmonary blood volume (116% of control) was observed after the race. Antimyosin scans were normal in 7 athletes and minimal antimyosin myocardial uptake was seen in 3. Transient alterations in biventricular performance present after the race correspond to function adaptation to strenuous exercise and are not due to irreversible myocyte damage.