The effects of the Ca2+-antagonist nimodipine on central dopamine (DA) neurons in mice were investigated in vivo. Nimodipine caused a dose-dependent decrease in the DA metabolite 3-methoxytyramine (3-MT) in striatum and the limbic region. If the brains were microwave radiated immediately after decapitation in order to minimize post-mortal accumulation of 3-MT, the effect of nimodipine was less pronounced and statistically not significant. Nimodipine markedly decreased the accumulation of 3-MT induced by pargyline, an inhibitor of monoamine oxidase, a phenomenon that was not attenuated by microwave radiation. Furthermore, whereas nimodipine had no effect on mouse motor activity when given alone it readily blocked the pargyline-induced increase in activity. The concentrations of 3,4-dihydroxyphenylacetic acid (DOPAC) in striatum and the limbic region were also reduced by nimodipine as was the accumulation of 3,4-dihydroxyphenylalanine (DOPA) measured after inhibition of the aromatic amino acid decarboxylase by 3-hydroxybenzylhydrazine (NSD 1015). In addition, nimodipine caused decreased concentrations of DA and homovanillic acid (HVA) in the limbic region but not in striatum. Nimodipine caused an increase in the striatal concentrations of 5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA); these changes were not seen in the limbic region. In conclusion, nimodipine appears to reduce the release as well as the synthesis of DA in mouse brain. These effects are believed to be related to the Ca2+-antagonism of nimodipine.