Previous studies have indicated that cAMP has bidirectional effects on epidermal growth factor (EGF)-induced DNA synthesis in cultured hepatocytes, acting to stimulate soon after plating (early G1) and to inhibit at later stages (nearer the G1/S transition). In this study we examined the role of the extracellular signal-regulated kinase (ERK) subgroup (p42/p44) of the mitogen activated protein (MAP) kinases both at growth-stimulatory and growth-inhibitory conditions. When added at low concentrations early during culturing, glucagon and 8-chlorophenylthio-cAMP (8-CPT-cAMP) did not increase MAP kinase activity, but enhanced the subsequent DNA synthesis. However, when administered at 24h, glucagon and 8-CPT-cAMP decreased basal and EGF-induced MAP kinase activity and also inhibited EGF-induced DNA synthesis. Thus, although MAP kinase might play a role in the growth-inhibitory effect, it does not seem to be involved in growth-promoting regulation by cAMP in hepatocytes.