The purpose of this study was to investigate the effects of daily exercise on susceptibility to sudden cardiac death. A 2 min coronary occlusion was initiated during the last minute of an exercise stress test and continued for 1 min after cessation of exercise in chronically instrumented dogs with a healed anterior wall myocardial infarction. Thirteen dogs developed ventricular fibrillation (VF; susceptible), while five did not (resistant). Before the exercise plus ischemia test, the baroreflex was evaluated with a bolus injection of phenylephrine (10 micrograms/kg). The changes in heart rate caused by a 30 mm Hg increase in systolic arterial pressure as well as the slopes of either heart rate or RR interval plotted against systolic arterial pressure were significantly lower in dogs that developed VR (resistant, -49.6 +/- 7.8; susceptible, -15.3 +/- 6.4 beats/min; p less than .001). Four resistant and eight susceptible animals were then placed on a 6 week daily exercise program, while eight susceptible dogs had an equal period of rest. At the end of the 6 week period the exercise plus ischemia test was repeated; no susceptible animal that performed daily exercise developed VF, and all but one of the rested animals did. Daily exercise improved baroreflex control of heart rate in the susceptible group but not in the resistant group. Rest did not alter baroreflex function (change in heart rate after 30 mm Hg increase in systolic arterial pressure: after 6 weeks of exercise, resistant -43.3 +/- 18.9 beats/min, susceptible -60.8 +/- 16.6 beats/min; after 6 weeks of rest, susceptible 27.4 +/- 11.0 beats/min). We conclude that daily exercise prevents VF induced by acute myocardial ischemia in a subpopulation of dogs that were previously identified as susceptible to sudden cardiac death. Exercise also altered the autonomic control of the heart, possibly decreasing sympathetic and/or increasing parasympathetic tone.(ABSTRACT TRUNCATED AT 250 WORDS)