The effect of lithium on protein kinase C (PKC) stimulation-induced serotonin and norepinephrine release facilitation was examined in [3H]5-HT and [3H]NE preloaded superfused rat brain slices. The facilitation of K+-evoked [3H]5-HT release elicited by the active phorbol esters PMA and PDBu was inhibited by 4 mM but not by 1 mM in vitro lithium. In experiments performed in cortical, hippocampal and hypothalamic slices obtained from animals treated for 3 weeks with lithium-containing diet, PMA-induced facilitation of K+-elicited [3H]5-HT release was found to be inhibited by the treatment (serum lithium less than 1 mEq/l). Basal [3H]5-HT efflux, which was enhanced by PMA in control animals, was also inhibited by lithium treatment. In parietal cortical slices, PMA elicited increase in K+-evoked [3H]NE release was prevented in slices taken from lithium-treated (3 weeks) animals. Lithium treatment did not affect the activity and distribution of protein kinase C in cortical tissue. However, 3 weeks of treatment reduced the PMA-induced translocation of the enzyme. These results suggest that lithium treatment interferes with serotonin and norepinephrine release facilitation which results from the stimulation of PKC by phorbol esters. These actions of the ion may be mediated by its ability to inhibit PMA induced PKC translocation.