Pigs were exercised for 5 min at five different treadmill speeds (1.0-1.8 m X s-1) (3 degrees incline), while oxygen consumption (MO2), carbon dioxide production (MCO2), and the electrocardiogram (ECG) were recorded continuously. Data were taken at rest, during exercise, and at 2, 5, 15, and 30 min after exercise. Values for MO2, MCO2, and heart rate (HR) showed progressive increases with increasing treadmill speed. The respiratory exchange ratio (R) increased during exercise and approached 1.0, but peak values were seen shortly after exercise. Heart rate, MO2, MCO2, and R reached steady-state values after 2 min of exercise, which were maintained for the duration of exercise. In most cases, these variables had returned to control levels 15 min after exercise. A high correlation between HR and MO2 was found in these animals. Prominent increases in T-wave amplitude of the ECG were associated with exercise and early recovery. The metabolic and cardiac changes associated with exercise in these animals were all qualitatively similar to responses seen in exercising humans. Thus, this study further supports the belief that the pig is a good model for studying the cardiopulmonary responses to exercise in humans.