Thirty dogs were subjected to infusions of protamine chloride 30 mg/min. Preceded by a pronounced drop in platelet count, there was 2.5–7 min after the start of the infusion a fall in aortic blood pressure, an increase in pulmonary arterial pressure, a decrease in lung compliance and in arterial oxygen tension. In heparinized animals (1000 U/kg) repeated infusions had the same effect as the initial infusion. Non-heparinized animals did not react significantly to the repeated infusions. The dose of protamine that elicited a reaction was not different in heparinized and in non-heparinized animals and it was lower than that calculated for neutralization of the heparin given. In 3 dogs defibrinogenated by Reptilase injections, the response to protamine was as strong as in dogs with normal fibrinogen levels. Fibrin formation is, therefore, excluded as a cause of the observed effects. Four dogs made refractory to protamine by repeated infusions, reacted to thrombin in the same manner as dogs not subjected to repeated infusions. Thus, protamine and thrombin act via different pathways, although the platelet as a common target cell is not thereby excluded. It is suggested that protamine exerts its haemodynamic and respiratory side effects via a release of smooth muscle stimulating substances from aggregated platelet