The effects of the beta-adrenoceptor blockers (BAB) alprenolol, atenolol, metipranolol, oxprenolol, practolol and propranolol on platelet aggregation stimulated by ADP, thrombin and collagen were investigated. The BAB were divided into three groups according to their liposolubility and antiaggregatory activity. The first group consists of the highly active drugs alprenolol and propranolol; metipranolol and oxprenolol are in the second group; the least active atenolol and practolol are in the third group. Collagen induced platelet aggregation was inhibited by BAB to the highest extent, followed by inhibition of thrombin stimulated aggregation. ADP-induced aggregation was inhibited to the lowest extent.Consistent with the rank order of their partition coefficients, BAB inhibited aggregation with the following order of potency: propranolol>alprenolol>metipranolol>oxprenolol>practolol>atenolol. A positive correlation was found between inhibition of thrombin and collagen stimulated platelet aggregation and the partition coefficients of the individual drugs. A significant time-dependent inhibition of platelet aggregation developed within 30 s of exposure of blood platelets to BAB.