Inhibition by atropine of the increased turnover of noradrenaline in the hypothalamus of rats exposed to cold.

1. Small doses of (-)-[(3)H] noradrenaline were injected into the lateral cerebral ventricles in rats to label radioactively the endogenous noradrenaline (NA) stores.2. Intraventricular injection of 25 mug atropine methonitrate at the same time inhibited the increased rate of disappearance of [(3)H] NA from the hypothalamus at an environmental temperature of 9 degrees C, when compared with the values at 24 degrees C, without impairing temperature regulation.3. At 32 degrees C, 25 mug atropine methonitrate caused a lethal hyperthermia. A dose of 5 mug was not lethal and did not inhibit the increased rate of disappearance of [(3)H] NA from the hypothalamus.4. It is concluded that the pathway which stimulates an increased turnover of NA in the cold contains an atropine sensitive synapse but is not the principal pathway of heat production. The increased turnover of NA in the heat probably does not involve an atropine sensitive synapse.