&NA;The effects of six months of a cardiac rehabilitation program, which included physical conditioning and dietary counseling, upon platelet aggregation at rest and following maximal exercise were evaluated in seven men who had survived a myocardial infarction. In platelet samples drawn at rest, aggregation induced with levoepinephrine (EPI) was strikingly reduced following the conditioning program in all subjects at all concentrations of EPI studied (10‐6M to 10‐3M). At peak EPI concentrations (10‐3M), primary wave aggregation fell from 40% ± 27% (mean ± SD) to 11% ± 5% (P + 0.0058), and secondary wave aggregation was abolished. Total aggregation, including the secondary wave, fell from 50% ± 26% to 11% ± 5% (P = 0.0113). No changes in the maximal extent of primary or secondary wave aggregation in response to adenosine diphosphate (ADP) or to collagen were noted, and there were no effects of a single bout of acute exercise upon platelet aggregation in response to any aggregating agent, either before or after the conditioning program. Possible mechanisms for reduced EPI sensitivity following this intervention include platelet alpha‐receptor desensitization, reduced cholesterol content of platelet membranes, or altered platelet kinetics.