The disappearance of labelled catecholamines formedin vivofrom14C‐tyrosine in the whole rat brain and in different regions of the rat brain has been studied. The elimination of the labelled amines from the brain followed first order kinetics between one and ten hours after14C‐tyrosine administration. The half‐lives were 2.9 hours for14C‐dopamine and 5.3 hours for14C‐noradrenaline. The rate of disappearance of14C‐dopamine was highest in the cerebral cortex and striatum and lowest in the cerebellum. The rate of disappearance of14C‐noradrenaline was highest in the cerebellum and lowest in the diencephalon. The use of the disappearance of labelled catecholamines as a measure of turnover of endogenous amines is discussed on the basis of results obtained with the tyrosine hydroxylase inhibitor H 44/68 and the dopamine‐β‐hydroxylase inhibitor FLA 63. It is suggested that FLA 63 accelerates noradrenaline turnover and also affects dopamine neurons in the brain. In accordance with previous studies on the accumulation of labelled catecholamines formed from14C‐tyrosine in rat brain regions, chlorpromazine accelerated the disappearance of14C‐dopamine in the striatum but had no significant effect in the other regions. The disappearance of14C‐noradrenaline was not influenced by chlorpromazine in any of the regions studied. The results support the view that chlorpromazine accelerates dopamine turn