Local inflammation, induced by s.c. injection of turpentine, evoked characteristic changes in the metabolism of antithrombin III, and alpha1-antitrypsin. For a period of approximately 36 h, the plasma half-lives of both protease inhibitors were shortened to 70--74% of the respective preinjection values. Similar changes were also observed in the slope of iodine-labelled albumin, suggesting that increased capillary permeability was primarily responsible for the losses of labelled proteins from the circulation. Incorporation of [3H]- or [14C]-leucine into albumin changed little during inflammation, but markedly increased values were measured for anti-thrombin III (3-fold), alpha1-antitrypsin (4-fold) and, above all, for fibrinogen (7-fold) 24 h and 48 h after the injection of turpentine. These changes in synthesis and elimination rates resulted in the following net balances: fibrinogen concentrations in plasma rose substantially during the early phase of inflammation; alpha1-antitrypsin concentrations increased gradually but to a significantly lesser extent, peak concentrations being reached after a reverse trend in fibrinogen concentrations had become apparent; antithrombin III concentrations remained steady throughout at levels which were only marginally above the pretreatment values.