1. Tyrosine hydroxylase is thought to be the rate limiting enzyme step in catecholamine biosynthesis. Inhibition of this enzyme using alpha-methyl-p-tyrosine resulted in a time dependent depletion (and repletion) of formaldehyde induced fluorescence in catecholamine-containing neurones of the central nervous system in developing and adult rats.2. Dopamine-containing neurones were depleted faster and more completely than noradrenaline-containing neurones.3. The extent of depletion caused by alpha-methyl-p-tyrosine in the initial 6-9 h period was more or less comparable in young and adult rats from the age of 1 week onwards; this suggests that catecholamine turnover increases with age and parallels the increase in catecholamine levels.4. The extent of depletion (and repletion) 18 h after administration of the inhibitor varied in animals of different age.5. Administration of a monoamine oxidase inhibitor just before administration of alpha-methyl-p-tyrosine resulted in a reduction of the extent of depletion caused by the latter drug, indicating that monoamine oxidase is important for the breakdown of catecholamines in rats of all ages.6. It is suggested that the catecholamine-containing neurones of the newborn are biochemically as well as functionally differentiated before completion of morphological differentiation.