1. Intrahypothalamic injection of either 5‐hydroxytryptamine (5‐HT) (20 μg) or tryptamine (1 μg) caused hypothermia and hyperthermia respectively in lightly restrained rats maintained at an ambient temperature of 20 ± 1 °C. 2. Both the 5‐HT‐ and the tryptamine‐sensitive sites were located within the same region of the preoptic area. 3. When rats were tested at different ambient temperatures (4, 20 and 29 °C), intrahypothalamic injection of 5‐HT caused a marked fall in core temperature (‐1·3 °C) in rats maintained at 4 °C, but smaller responses were obtained at 20 and 29 °C (‐0·9 and ‐0·5 °C respectively). Tryptamine caused a significant hyperthermia in rats kept at 20 °C, but had no significant effect in rats maintained at either 4 or 29 °C. 4. The hypothermic effect of 5‐HT was selectively antagonized by systemic pre‐treatment with cyproheptadine (2·5 mg/kg), but not by methergoline (0·625 mg/kg) and methysergide (0·2 mg/kg). In contrast, the hyperthermic effect of tryptamine was blocked by methergoline and methysergide, but not by cyproheptadine. 5. Cyproheptadine (2·5 mg/kg) reduced the ability of rats to cope with a heat load but had no effect on the response to cold. In contrast, methergoline (0·625 mg/kg) and methysergide (0·2 mg/kg) reduced the ability to cope with cold but the rats' ability to cope with a heat load remained intact. 6. These results suggest the existence of two indoleamine pathways within the preoptic anterior hypothalamus involved in the control of body temperature: a serotonergic pathway mediating heat loss and a non‐serotonergic pathway mediating heat gain. The non‐serotonergic system may exert its effects by modulating the activity of a central serotonergic system.