Mechanisms of tryptophan (a 5-HT precursor)-induced changes in body temperature were investigated in rats pretreated with pargyline, a monoamine oxidase inhibitor (MAO-I). Tryptophan (100 mg/kg, i.p.) did not affect the body temperature in rats, but it produced significant hypothermia followed by marked hyperthermia and higher mortality in the pargyline-pretreated rats. 5-HT depletion with p-chlorophenylalanine (p-CPA, 100 mg/kg/day for 3 days) significantly suppressed not only the body temperature change but also the mortality and 5-HT syndrome following tryptophan plus pargyline administration. Although propranolol (10 mg/kg, i.p.), a beta-adrenoceptor antagonist, did not alter the hypothermia caused by tryptophan in the pargyline-pretreated rats, pindolol (2 mg/kg, S.C.), a 5-HT1A receptor and beta-adrenoceptor antagonist, suppressed the hypothermia but not the hyperthermia or mortality caused by the same treatment. On the other hand, spiperone and ketanserin, 5-HT2 receptor antagonists, at doses of 3 mg/kg, potentiated the hypothermia and completely suppressed the hyperthermia and mortality caused by tryptophan in the pargyline-pretreated rats. These results suggest that tryptophan-induced hypo- and hyperthermia are mediated by 5-HT1A and 5-HT2 receptors, respectively, in the pargyline-pretreated pretreated rats.