Tyrosine and tryptophan hydroxylase activity was studied in the postnatal rat brain in vivo by measuring the accumulation of dihydroxyphenylalanine and 5-hydroxytryptophan, respectively after inhibition of L-aromatic amino acid decarboxylase with NSD 1015. With increasing age there was a significant increase in the amount of dopa and 5-HTP accumulated in the brain after administration of NSD 1015. After 30 min in a 12% oxygen environment there were significant reductions of tyrosine hydroxylase and tryptophan hydroxylase activity at 1, 14 and 28 but not 4 days of postnatal age. Further, the decrease in 5-HTP accumulation was significantly more marked at 14 and 28 days than at 1 day of age. Thus, the oxygen-dependent synthesis of the neurotransmitter 5-hydroxytryptamine seems to be less vulnerable in the early postnatal rat brain.