1. Parallel series of experiments were carried out in the rat and mouse in order to investigate the mechanism(s) underlying the hypothermia induced in rodents by the selective 5-HT1A receptor agonist, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT). 2. In the mouse, lesioning of central 5-hydroxytryptaminergic neurones (by use of the neurotoxin, 5,7-dihydroxytryptamine; 5,7-DHT) abolished the hypothermic response to 8-OH-DPAT, and depletion of brain 5-hydroxytryptamine (5-HT) levels (with the 5-HT synthesis inhibitor, p-chlorophenylalanine) markedly attenuated the response in this species. These pretreatments did not significantly attenuate 8-OH-DPAT-induced hypothermia in the rat, except for a significant attenuation of the response in 5,7-DHT-lesioned rats at the top dose of 8-OH-DPAT (1.0 mg kg-1, s.c.). 3. Pharmacological pretreatments which facilitate 5-HT release (selective 5-HT uptake inhibitors, precursor (5-hydroxytryptophan) loading, or fenfluramine), markedly attenuated or abolished 8-OH-DPAT-induced hypothermia in the mouse. These pretreatments generally had no significant effect on 8-OH-DPAT-induced hypothermia in the rat. 4. The selective noradrenaline uptake inhibitor, desipramine, had no effect on the hypothermic response to 8-OH-DPAT in either species. The selective dopamine uptake inhibitor, nomifensin, significantly increased the hypothermic response to 8-OH-DPAT in the mouse, but did not affect the response in the rat except at high, motor stimulant doses, when the response was attenuated. 5. These data are consistent with the hypothesis that 8-OH-DPAT-induced hypothermia is mediated by presynaptic autoreceptors in the mouse and by postsynaptic 5-HT1A receptors in the rat. Preliminary data also indicate an involvement of dopamine release in the mouse but not in the rat.