Generation of thrombin and activated protein C in the inflammatory focus was demonstrated in rats with experimental acute peritonitis. The contents of thrombin and activated protein C peaked by the 30th and 120th minute of inflammation, respectively. In vitro study showed a decrease in spontaneous and compound 48/80-induced secretion of beta-hexosaminidase by peritoneal mast cells under the influence of activated protein C in low concentrations. The antiinflammatory effect of protein C in the focus of acute peritonitis is probably realized through NO release from peritoneal mast cells. This conclusion is derived from the data that L-NAME abolishes the protective effect of activated protein C.