Abstract 1. 1. Rats reared from birth at 30°C show a permanent deficit in body temperature regulation. 2. 2. To test the ability of heat-reared rats to respond to an adrenergic drive, carbon dioxide production was measured at fives doses of noradrenaline in rats reared at either 20 or 30°C. 3. 3. Noradrenaline-induced carbon dioxide production was greater at all doses in 20°C rats, but sensitivity to noradrenaline was the same in heat-reared and control animals. 4. 4. These findings support the conclusion that the thermoregulatory deficit induced by heat rearing is due to a change induced in the nervous system.