The membrane potential of platelets suspended in physiological medium and membrane potential changes induced by high potassium concentrations, ouabain and cooling have been measured using a cyanine fluorescent dye (3,3'-dipropylthiodicarbocyanine) [corrected]. The membrane potential of platelets suspended in physiological medium was -63.8 mV. High potassium concentrations, ouabain and cooling induced depolarization of platelet membrane. Depolarization using the above procedures enhanced platelet aggregation induced by ADP, adrenaline and collagen. These results suggest that the membrane potential could modulate platelet activity.