OBJECTIVE:To investigate the pharmacological mechanisms underlying the induction of thermogenesis by Metabolite 2 (M2; BTS 54 505), a major pharmacologically active metabolite of the anti-obesity drug, sibutramine. DESIGN: Adult female Wistar rats were treated with M2 or vehicle, with or without various monoamine receptor antagonists, prazosin, RS79948, metergoline, propranolol and ( þ )butaclamol. MEASUREMENTS: Colonic temperature and food intake at room temperature (21 AE 1 C), thermoregulatory behavioural response, operant responding for exogenous heat at 7 8 C and oxygen consumption at thermoneutrality (29 C). RESULTS: M2 (10 mg=kg, p.o.) significantly increased colonic temperature during the 4.5 h period following drug administration. This effect was abolished by the non-selective 5-HT receptor antagonist, metergoline (1 mg=kg, p.o.), and a 1 -adrenoceptor antagonist, prazosin (1 mg=kg, p.o.), measured at 1.5 -2.5 h post-M2 administration, and was partially antagonized by each antagonist at 3.5 -4.5 h. The non-selective b-adrenoceptor antagonist, propranolol (1 mg=kg, p.o.), had no effect on the M2-induced increase in colonic temperature, whereas at 20 mg=kg (p.o.), propranolol partially inhibited the effect of M2 on colonic temperature. By contrast, the selective a 2 -adrenoceptor antagonist, RS79948 (1 mg=kg, p.o.), and the D2=D1 receptor antagonist, ( þ )butaclamol (200 mg=kg, p.o.), did not alter the effect of M2 on colonic temperature. In the thermoregulatory study, M2 (10 mg=kg, i.p.)-treated rats required significantly less radiant heat at 7 8 C to maintain body temperature, and this effect was not affected by the D2=D1 receptor antagonist ( þ )butaclamol (100 mg=kg 71 , i.p.). The hypophagia induced by M2 (10 mg=kg) measured up to 24 h was partially antagonized by the a 1 -adrenoceptor antagonist, prazosin, whereas metergoline, RS79948, propranolol and ( þ )butaclamol had no effect on M2-induced hypophagia. CONCLUSION: It is concluded that 5-HT, a 1 -and b 3 -adrenoceptors are involved in the induction of thermogenesis by M2, whereas the hypophagic effect is mainly mediated via a 1 -adrenoceptors. These findings are consistent with M2 increasing 5-HT and noradrenaline tone via potent reuptake inhibition which subsequently results in increased efferent sympathetic activity to brown adipose tissue (BAT).