Platelet aggregation appears to play a prominent role in myocardial ischemia. Verapamil, a slow-channel blocking agent with important antiarrhythmic and vasodilating actions, has been shown to inhibit in vitro platelet aggregation. We used an electronic particle size analyzer to evaluate the effects of verapamil on platelet aggregation in vitro and in vivo in 88 rats. The intravenous injection of verapamil (0.4 mg/kg) did not change the platelet count compared to control animals receiving an equal volume of normal saline (verapamil, 1.1 +/- 0.04 x 10(6)/mm3, vs. control, 1.2 +/- 0.09 x 10(6)/mm3, (p greater than 0.05). The mean size of platelet aggregates induced by adenosine diphosphate (0.2 microM), was reduced by verapamil (verapamil, 15.3 +/- 1.2 x 10(3) micron3 vs. control 24.4 +/- 2.7 x 10(3) micron; p less than 0.01). Platelet aggregates induced in vivo, following a standardized technique of extravasation of right iliac artery blood into the peritoneal cavity, were also smaller following verapamil infusion (verapamil, 12.6 +/- 1.1 x 10(3)micron3, vs control, 17.3 +/- 0.9 x 10(3) micron3 p less than 0.001). We conclude that verapamil exerts and inhibitory effect on platelet aggregation both in vitro and in vivo. This property may add an important new dimension to its potential therapeutic usefulness in ischemic heart disease.