Protamine is given after cardiopulmonary bypass to reverse the effect of heparin. However, the adverse effects of an overdose have not been fully evaluated. In this canine study, a standard dose of one bolus of heparin (300 IU per kilogram of body weight) was neutralized by an infusion of protamine hydrochloride in a 1:2 ratio. Platelet number and function were severely reduced by this overdose of protamine, and this finding correlated with a prolongation of the bleeding time. Giving a titrated dose of protamine hydrochloride to reverse heparin activity, measured by the activated clotting time, affected neither platelet number nor function. When prostacyclin (0.5 microgram/kg/min) was infused during administration of an overdose of protamine, platelets again were not affected by the excess of protamine.