Intracerebroventricularly administered neurotensin and [Gln4]-neurotensin (50-200 microgram) increased the formation of Dopa in different brain regions of rats after inhibition of the aromatic L-amino acid decarboxylase. For both neuropeptides these increases were dose dependent (20-150%). In the corpus striatum [Gln4]-neurotensin was twice as active as neurotensin and it tended to be more active also in other brain regions. The brain tyrosine concentrations were also increased. [Gln4]-neurotensin (100-200 microgram) following inhibition of the aromatic L-amino acid decarboxylase, increased the accumulation of 5-hydroxytryptophan in all brain regions by 30-60%. In contrast, neurotensin was completely inactive. In both cases the brain tryptophan concentrations were increased. Both neurotensin and [Gln4]-neurotensin also accelerated the disappearance of dopamine, noradrenaline and 5-hydroxytryptamine after inhibition of monoamine synthesis. These results show an increased brain monoamine turnover induced by both neuropeptides.