The fever elicited by 10 micrograms/kg intravenous E. coli endotoxin was significantly higher in capsaicin desensitized rats than in controls, but there was an upper limit to the rise in body temperature in both groups. Capsaicin desensitization permitted the participation of various heat producing mechanisms even if the initial body temperature was high. The fever course was characteristically biphasic, similar as in control rats, thus desensitization did not alter the factors mediating the various phases of the endotoxin response.