Cardiac fibroblasts play a critical role in the process of pathophysiological cardiac hypertrophy as the cell type responsible for fibrosis. Whereas many growth factors and hormones are thought to be involved, possible crosstalks between signal transduction pathways are not well defined. Therefore we investigated the influence of cAMP- and cGMP-dependent protein kinases (cAK, CGK) on platelet derived growth factor (PDGF) stimulated growth of primary cardiac fibroblasts from adult rats. We show here that PDGF-BB induced cell proliferation can be inhibited by activation of the endogenous cAK directly via the cAMP analog 5,6-DCL-cBIMPS as well as indirectly via the cAMP-elevating receptor agonist prostaglanding-E₁ (PGE1). In contrast, activation of the endogenous cGK-I has no influence on cardiac fibroblast cell growth. The strength of the proliferation inhibition is dependent on the time course of cAK activation, i. e., longer activation with the cAMP analog results in stronger proliferation inhibition. No significant influence of cAK or cGK-I on Akt activation or on the short-term activation of the MAPK cascade was observed. In contrast, 5,6-DCI-cBIMPS treatment of cardiac fibroblasts causes an inhibition of long-term MAPK phosphorylation. A prolonged PGE1-dependent cAMP signal after addition of 3-isobutyl-1-methyl-xanthine (IBMX) blocks the second long-term MAPK phosphorylation as well. Therefore the suppression of MAPK long-term phosphorylation, four to eight hours after PDGF-BB stimulation, appears to play the major role in inhibition of cardiac fibroblast proliferation by cAK activators.