Ketorolac tromethamine is a potent prostaglandin synthetase inhibitor useful in the treatment of postoperative pain. Since it is also known to have antiplatelet properties, we determined the effect of ketorolac, alone and in combination with low-dose heparin, on hemostasis. Each of 12 healthy male volunteers received the following drug combinations on a double-blind, crossover basis: ketorolac dummy/heparin dummy, ketorolac active/heparin dummy, ketorolac active/heparin active, and ketorolac dummy/heparin active. Ketorolac significantly prolonged bleeding time, and inhibited platelet aggregation and platelet thromboxane production. Heparin had no effect on bleeding time or platelet function, but significantly prolonged the kaolin-cephalin clotting time and increased anti-Xa levels. Ketorolac had no effect on the kaolin-cephalin clotting time or anti-Xa levels, and no interaction was found between ketorolac and heparin. The modest prolongation of bleeding time with ketorolac is unlikely to be of any major clinical significance, as the value remained within the normal range in almost all subjects. However, because of its antiplatelet properties, the drug should be used with caution in persons with hemostatic disorders.