Dopamine (DA) synthesis in rat striatal synaptosomes was approximately doubled either by treating the animals from which the synaptosomes were obtained with reserpine, or by treating the preparations in vitro with d-amphetamine, ouabain or dibutyryl cyclic AMP. The concentration-response curve of DA synthesis inhibition by apomorphine was shifted to the right after treatment with all these compounds. The inhibitory effect of bromocriptine on DA synthesis was reduced completely after treatment with all the above compounds with the exception of dibutyryl cyclic AMP. When the inhibitory effect of bromocriptine was eliminated by treatment with reserpine or d-amphetamine, bromocriptine antagonized the inhibitory effect of apomorphine. This indicates that bromocriptine could still be bound to the DA autoreceptors and that the reduced sensitivity was due to a reduced functioning of the DA autoreceptors. The reduced sensitivity to apomorphine observed after all the above treatments was possibly due both to a reduced function of and/or to a reduced binding to the DA autoreceptors. The increase in DA synthesis produced by treatment with reserpine in vivo or with d-amphetamine or ouabain in vitro was additive to that produced by a maximally effective concentration of dibutyryl cyclic AMP in vitro, and thus mediated by a presumably non-cyclic AMP-dependent mechanism. Our results obtained with bromocriptine suggest that stimulation of the DA autoreceptors may inhibit DA synthesis by diminishing Ca2+-dependent and not cyclic AMP-dependent phosphorylation of tyrosine hydroxylase.