Coronary blood flow might be reduced by platelet aggregates or by vasospasm induced by platelet-produced thromboxane A2. Therefore the effects of the platelet inhibitor ticlopidine (500 mg daily) on platelet function and on exercise tolerance were investigated in a double-blind placebo-controlled study in 38 middle-aged men with stable incapacitating angina pectoris. Before and after 4 and 8 weeks of treatment, exercise tests were performed in warm and cold environments. The in vitro platelet reactivity to ADP was determined at rest and the plasma levels of beta-thromboglobulin (BTG) and platelet factor 4 (PF4) were measured before and immediately after exercise. There were no signs of increased platelet activity at rest or after exercise as judged by the levels of BTG and PF4. Despite a potent inhibition of platelet reactivity to ADP in vitro during ticlopidine treatment, the exercise tolerance was reduced in exercise tests in both warm and cold environments and in daily life. Therefore platelet activity does not seem to play any significant role in exercise tolerance in the stable phase of angina pectoris.