We have earlier found that in Jurkat cells activation of protein kinase C (PKC) enhances the cyclic adenosine monophosphate (cAMP) accumulation induced by adenosine receptor stimulation or activation of Gs. Here we have therefore examined the effect of the phorbol ester PMA (phorbol 12-myristate 13-acetate) which stimulates PKC and a combination of the adenosine receptor agonist NECA (5'-(N-ethyl)-carboxamido adenosine) and forskolin to raise cAMP, on the levels of c-Fos and Jun and on the binding and transcriptional activity of the transcription factor, activator protein-1 (AP-1). PMA treatment caused a concentration- and time-dependent increase in both c-Fos and Jun immunoreactivity in contrast to cAMP elevation that had only a slight effect. Both PMA and the combination of NECA and forskolin acted together either to increase (c-Fos) or decrease (Jun) protein levels as well as increasing AP-1 binding, as judged by gel-shift assay, and AP-1 transcriptional activity. Furthermore there was a clear-cut synergy between the PKC stimulator and the cAMP elevating agents. The results demonstrate that the simultaneous activation of PKC and elevation of cAMP leads to an enhanced AP-1 transcriptional activity in a T-leukemia cell line, suggesting that the previously observed interaction between the parallel signal transduction pathways may have functional consequences at the level of gene transcription.