The results of the present experiments show that local microinjections of Arg8-vasopressin into the nucl. caudatus cause an increase in the alpha-methyl-p-tyrosine methylester-HCl-induced disappearance of dopamine (DA) at the site of administration of the peptide. It is suggested that the caudate nucleus is the site of action of the peptide with respect to its effect on nigrostriatal DA neurons. This conclusion is corroborated by both the finding that microinjection of Arg8-vasopressin into the A9 region, which contains the cell bodies of the nigrostriatal system, was ineffective, and the results of push-pull experiments which showed an enhancement in apparent DA release in the nucl. caudatus when Arg8-vasopressin was co-perfused through the cannula system. Arg8-vasopressin appears to have a rather modest effect on nucl. caudatus DA synthesis, as was deduced from the results of experiments in which the in vitro conversion of tritiated tyrosine into tritiated DA was measured following in vivo Arg8-vasopressin administration as well as after in vitro incubation with the peptide. In conclusion, the interaction of vasopressin with the nigrostriatal DA system appears to be at the level of the DA terminals in the nucl. caudatus rather than at the level of the substantia nigra, and secondly, Arg8-vasopressin appears to affect DA release in the nucl. caudatus rather than DA synthesis.