The effect of total alkaloid extracted from Peganum harmala seeds collected in Egypt on body temperature was studied in rats. Intraperitoneal administration of the Peganum harmala extract produced significant and dose-dependent hypothermia. Similarly, harmine and harmaline, major constituents of the harmala alkaloid, lowered the body temperature. Pretreatment with p-chlorophenylalanine (100 mg/kg/day for 3 days), a 5-HT synthesis inhibitor, significantly attenuated the hypothermic effect of the total alkaloid and harmine, while it tended to block the hypothermic action of harmaline. Methysergide (2 mg/kg), a 5-HT antagonist, significantly attenuated the hypothermia induced by harmala alkaloids. Pindolol (0.05-2 mg/kg), a 5-HT1A receptor and beta-adrenoceptor antagonist, partly blocked the hypothermic effect of the harmala alkaloids in a dose-dependent manner, whereas propranolol (10 mg/kg), a beta-adrenoceptor antagonist, failed to alter it, suggesting that beta-adrenoceptor is not involved in the hypothermia caused by the alkaloids. Pretreatment with a dopamine receptor antagonist haloperidol (5 mg/kg, s.c. and 2 mg/kg, i.p. 24 and 2 h before the experiment, respectively) significantly attenuated the hypothermic effect of harmala alkaloids. Moreover, in haloperidol pretreated rats, methysergide (2 mg/kg, i.p.) and pindolol (0.05 and 2 mg/kg) completely attenuated the hypothermic effect of the alkaloids. These data suggest that harmala alkaloids produce hypothermic effect mainly through endogenous 5-HT stimulation of 5-HT1A receptor.