The relative sensitivities of platelet and fibrinogen production to endotoxin were determined in male New Zealand rabbits. E. coli endotoxin was administered in single intravenous doses of 0.1 to 50.0 mu/g/kg body mass. 75SeM was injected 5, 12, or 18 hr after endotoxin, and the percent incorporation into platelets and fibrinogen was used to measure thrombopoiesis and fibrinogen synthesis. Leukopenia occurred after the infusion of endotoxin at all dose levels; the lowest dose that caused thrombocytopenia was 0.5 microgram/kg. Endotoxin was detected with the Limulus test in plasma of animals that received 5 to 50 microgram/kg. The stimulation of fibrinogen and platelet production, as well as the postinfusion decrease in platelet count, correlated directly with the log of the dose of endotoxin infused. The lowest dose of endotoxin that stimulated platelet and fibrinogen production was 0.5 microgram/kg. In animals that received this dose of endotoxin, increased fibrinogen production was detected only when 75SeM was injected 5 hr later. In animals injected 5, 12, or 18 hr later. Increased platelet production was detected at these two doses only when 75SeM was injected 18 hr after endotoxin. The data indicate that fibrinogen and platelet production have similar sensitivities to endotoxin, although the time course of stimulation is different for these two blood components.