The stimulation of dopamine synthesis in rat brain striatal synaptosomes produced by the depolarizing agent veratridine was markedly reduced by prior in vivo amphetamine administration. This effect did not appear to be due to an interference with the depolarization process, per se, since veratridine-induced inhibition of tyrosine uptake, a biochemical correlate of depolarization, was unaffected by amphetamine. Antagonism of veratridine-induced synthesis stimulation was not duplicated by in vivo treatment with pargyline, apomorphine, gamma-butyrolactone or haloperidol, suggesting that this effect may be due to a direct cellular action of amphetamine. In contrast to the inhibition of veratridine-induced synthesis stimulation produced by in vivo amphetamine administration, the synthesis stimulation produced by in vitro amphetamine treatment was not reduced. However, this stimulation was altered in character and was no longer calcium-dependent. A similar loss of calcium-dependency for amphetamine-induced synthesis stimulation was also observed after in vivo reserpine treatment. These results suggest that in vivo amphetamine administration can markedly alter the interactions between tyrosine hydroxylase and synaptosomal dopamine pools that may be involved in the regulation of catecholamine formation.