The effect of activated human Protein C (PCa) infusion on the coagulation and fibrinolytic systems of the cat was examined. Cats received bolus i.v. injections of PCa of 3 micrograms/mL (n = 7), 10 micrograms/mL (n = 2) or 16 micrograms/mL (n = 1) of calculated total blood volume. Control cats (n = 7) received either unactivated Protein C (10 micrograms/mL) or the activation vehicle containing heparin and antithrombin in tris-saline buffer. Citrated blood samples were drawn prior to infusion and 5, 10, 20, 40, 60, 120 and 180 minutes post infusion. PCa caused an immediate, dose dependent increase in anticoagulation as measured by APTT which began to normalize within 20 minutes. PCa also increased fibrinolytic activity measured by euglobulin clot lysis time and an 125I whole blood clot lysis assay. However, the profibrinolytic effect of PCa did not parallel the anticoagulant effect. In all treated cats, maximum fibrinolytic activity did not occur until 40-120 minutes after infusion. Control cats demonstrated no significant change in coagulation or fibrinolytic activity. Our results demonstrate that infusion of activated human Protein C can induce anticoagulation and fibrinolytic activity in the cat.