The influence of varying brain levels of dopamine, noradrenaline and 5-HT on their respective synthesis rates has been investigated. The first step in monoamine synthesis was studied in vivo by measuring the accumulation of dopa and 5-hydroxytryptophan after inhibition of the aromatic L-amino acid decarboxylase. Variations in monoamine levels were obtained by combined treatment with inhibitors of the decarboxylase (NSD 1015 or Ro 4-4602) and of monoamine oxidase (pargyline). An increase in monoamine levels by pargyline was found to inhibit the synthesis of dopamine, noradrenaline and 5-HT. Conversely, a decrease in monoamine levels induced by the decarboxylase inhibitor Ro 4-4602 appeared to stimulate dopamine and noradrenaline synthesis but had no effect on 5-HT synthesis. The influence of varying levels of dopamine and noradrenaline on the synthesis of these amines could still be demonstrated after blockade of dopamine receptos and of alpha-adrenergic (noradrenaline) receptors by haloperidol, suggesting that the mechanism involved in this feedback control is mediated via end-product inhibition of tyrosine hydroxylase. On the other hand, the stimulating influence of haloperidol on the synthesis of catecholamines does not seem to be directly related to changes in catecholamine levels. It is concluded that the short-term control of catecholamine synthesis presumably involves two independent feedback mechanisms, one intraneuronal mechanism operating via end-product inhibition, and one synaptic mechanism mediated via dopamine and noradrenaline receptors, respectively. Both pre- and postsynaptic receptors may be involved in the latter mechanism.