Ten male patients suffering from stable angina pectoris were studied at rest and immediately after exercise during treatment either with timolol (a non-selective beta-blocker) or with metoprolol (a beta 1-selective blocker). Timolol induced a significant increase in platelet aggregation and a reduction in platelet cyclic AMP, and it also raised the plasma adrenaline at rest and during exercise as compared to the pre-treatment level. Metoprolol had none of these effects. Prior to medication and during metoprolol treatment, exercise led to an increase in the peripheral platelet count, whereas timolol was associated with a reduction of platelets during physical effort. Neither drug affected platelet thromboxane B2 at rest. During exercise, its level was not affected in the pre-treatment period or during metoprolol treatment but it was sharply increased by timolol therapy.