Macusine B reduced the induction time and potentiated the barbiturate sleeping time of mice, possibly as a consequence of the hypothermia produced by the alkaloid. There was no evidence to suggest that macusine B affects the synthesis or metabolism of noradrenaline, dopamine, 5‐hydroxytryptamine or γ‐aminobutyric acid in the rat brain, nor was there any evidence to suggest that it affected the binding of noradrenaline and 5‐hydroxytryptamine within the brain. There is an indication that macusine B may act on central adrenergic receptors. The slight depression of the behavioural activity which occurs in rats and mice after the injection of macusine B was probably due to the hypothermia induced by the al