Phorbol-12,13-dibutyrate (PDiBu) augmented the accumulation of [3H]-cyclic AMP in rat hippocampal slices elicited by the adenosine receptor agonist 5'-(N-ethyl)-carboxamidoadenosine (NECA). A similar, but less pronounced, effect was observed when using tetradecanoylphorbol acetate (TPA) provided that the slices were preincubated with the phorbol ester before addition of NECA. PDiBu also induced cyclic AMP accumulation in the absence of NECA, but this effect could be markedly reduced or even abolished by the adenosine antagonist 8-para sulphophenyl theophylline (8-pst). Whereas no clearcut synergism was observed between isoprenaline and PDiBu under normal circumstances, such synergism was observed after elimination of the cyclic AMP accumulation caused by endogenous adenosine. PDiBu slightly enhanced the stimulatory effect of low (0.1-1 microM) concentrations of forskolin, but did not enhance the effect of 10 microM forskolin. In the presence of forskolin, another adenosine analogue, R-PIA, had a biphasic effect on cAMP accumulation. With PDiBu present the inhibitory phase was entirely eliminated and the stimulatory phase substantially enhanced. A synergistic interaction between PDiBu and NECA was found even after inhibition of the Ni-protein by N-ethyl maleimide. It is concluded that the phorbol ester may enhance the stimulatory effect of adenylate cyclase stimulatory compounds, including endogenous adenosine. The effect may partly depend on protein kinase C-mediated inactivation of N-protein mediated adenylate cyclase inhibition, but other mechanisms also seem to be involved.