TYROSINE hydroxylase (tyrosine-3-monooxygenase), presumably the rate-limiting enzyme in the biosynthesis of the adrenergic transmitters dopamine and noradrenaline1,2, catalyses the hydroxylation of both phenylalanine and tyrosine3,4. This property has been exploited for the selection of adrenergic-like mouse neuroblastoma cells5, which have high tyrosine hydroxylase activity and can grow in the absence of tyrosine because they can convert sufficient phenylalanine to tyrosine for cell growth. N1E-115 is such a neuroblastoma clone5,6 and we have now found that, when the amount of tyrosine in the medium is increased, there is an increase in the amount of tyrosine hydroxylase in N1E-115 cells. Modification of the amount of this rate-limiting enzyme by its substrate concentration may play a fundamental role in the regulation of the biosynthesis of catecholamines.