This study investigated the effects of chlorpheniramine (CPA, 10-25 mg/kg), diphenhydramine (DIPH, 20 mg/kg), tripelennamine (TRIP, 20 mg/kg), and pyrilamine (PYRI, 20 mg/kg) on 3, 4-methylenedioxymethamphetamine (MDMA, 20 mg/kg x 2)-induced hyperthermia and depletion of indoles in rat brains, on the uptake of serotonin and dopamine into rat synaptosomes, on the binding affinity of CPA for biogenic amine transporters in the synaptosomes of rat brain, and on the scavenging hydroxyl free radicals activity. Rats were treated with two injections of MDMA, CPA, DIPH, TRIP, PYRI, and saline, alone or in combination of MDMA with one of the antihistamines, 6 h apart and sacrificed 5 days later. Rectal temperature was measured prior to and hourly following the drug injections for 13 h. As compared to saline controls, MDMA increased body temperature and decreased levels of indoles, measured by HPLC, in several brain regions of rats. CPA attenuated and DIPH had no effect on MDMA-induced hyperthermia, yet both attenuated the depletion of indoles, whereas PYRI and TRIP potentiated these effects. CPA inhibited the binding of [(3)H]paroxetine and [(3)H]nisoxetine to the synaptosomes of cerebral cortex and of [(3)H]win 35,428 to the synaptosomes of striatum. CPA, DIPH, TRIP, and PYRI inhibited [(3)H]serotonin uptake. CPA, PYRI, and TRIP, but not DIPH, scavenge hydroxyl radicals. Possible mechanisms of the different effects of the antihistamines on MDMA-induced hyperthermia and depletion of serotonin are discussed. Published 1999 Wiley-Liss, Inc.