Plasma concentrations of 6-oxo-prostaglandin F1 alpha (6-oxo-PGF1 alpha) and thromboxane B2 (TXB2) were measured by radioimmunoassay in arterial blood before and after injections of the complement activator, cobra venom factor (CVF). During the control period, the concentration of 6-oxo-PGF1 alpha, which gives the sum of prostacyclin plus 6-oxo-PGF1 alpha, and TXB2 were, respectively, less than 20 pg ml-1 and 70 +/- 15 pg ml-1. Intravenous injections of CVF induced dose-dependent, reversible elevations in the plasma levels of both prostanoids. The time courses for the increases of 6-oxo-PGF1 alpha and TXB2 paralleled the arterial hypotension and thrombocytopenia, suggesting the existence of a causal relationship between these parameters. The results further support our hypothesis that complement-dependent formation of arachidonic acid metabolites contributes to some of the haemodynamic and haematological changes occurring during endotoxin shock.