Five instrumented and eight noninstrumented dogs were progressively trained for 12-18 wk on a motor-driven treadmill. Data were compared with 14 instrumented and 8 noninstrumented control dogs. Gastrocnemius malate dehydrogenase activity was significantly increased in the trained dogs (887 +/- 75 vs. 667 +/- 68 mumol . g-1 . min-1). The trained dogs also showed significant increases in maximum work capacity, cardiac output (7.1 +/- 0.5 vs. 9.1 +/- 0.7 1/min), stroke volume (25.9 +/- 2.0 vs. 32.0 +/- 2.0 ml/beat), and left ventricular (LV) positive dP/dtmax (9,242 +/- 405 vs. 11,125 +/- 550 Torr/s). Negative dP/dtmax was not significantly different. Peak LV systolic pressure increased with exercise, but there was no significant difference between the trained and control dogs. LV end-diastolic pressure did not change with exercise and was the same in both groups. Tension-time index was lower in the trained dogs at rest and submaximum exercise (9.7 km/h, 10%) but was not different at maximum exercise. Diastolic pressure-time index was significantly higher in the trained dogs at rest and during submaximum exercise but was not different at maximum exercise. LV coronary blood flow was significantly reduced at rest (84 +/- 4 vs. 67 +/- 6 mo . min-1 . 100 g-1) and during submaximum exercise (288 +/- 24 vs. 252 +/- 8 ml . min-1 . 100 g-1). During maximum exercise flow was not significantly different (401 +/- 22 vs. 432 +/- 11 ml . min-1 . 100 g-1) between the control and trained groups. The maximum potential for subendocardial flow was unchanged with training despite the development of mild hypertrophy.