The effect of thrombin on the oxygen consumption of washed human platelets was measured polarographically with the Clark oxygen electrode. The average basal respiratory rate was 18 plus or minus 1.6 (mean plus or minus S.E.) natoms oxygen per min per 10-9 platelets. Thrombin (1.9 units/ml) caused a 4-13-fold increase in the rate of oxygen consumption (138 plus or minus 14 (mean plus or minus S.E.) natoms oxygen per min per 10-9 platelets). The thrombin-stimulated increase of oxygen consumption was transient, lasting from 1 to 1.5 min before returning to the respiratory rate observed before the thrombin addition. Release of platelet constituents appeared to precede the stimulation of oxygen consumption. These results may provide a basis for explaining the discrepancy in the literature concerning the effects of thrombin on platelet respiration.