We have shown previously that administration of endotoxin induces a smaller decrease of body temperature in spontaneously hypertensive rats (SHR) than in normotensive Brown Norway (BN) rats. Several studies have suggested that tumor necrosis factor alpha (TNFalpha) is one of the mediators of the body-temperature response to endotoxin. To test whether the TNFalpha gene could be involved in determination of the observed difference in the body-temperature response to endotoxin, we studied SHR (n = 6) and a congenic strain, SHR.1N (n = 5), which differs from SHR by a segment of chromosome 20 originating from BN and containing the TNFalpha gene. Body temperature was recorded continuously by means of radiotelemetry. We showed that, in both strains, an intraperitoneal injection of endotoxin (500 microg/kg of body weight) induces a rapid hyperthermic phase (20-40 minutes post-injection), which is followed, first, by a hypothermic phase (100-120 minutes post-injection) and, then, by a late hyperthermic phase (seven hours). Although both strains demonstrated a similar trend in the response, a significant difference was observed between the two response curves (P = 0.0001). Further analysis at each time point revealed that the two strains differed significantly at a peak of the hypothermic phase (P = 0.035) and the late hyperthermic phase (P = 0.035). In conclusion, these data indicate that the differential chromosomal segment of SHR.1N contains a gene(s) causally related to the body-temperature response to endotoxin. In the light of previously published data, the TNFalpha gene appears to be the most likely candidate gene within the segment.