Expression of the proto-oncogenes c-fos and c-jun was analysed in the insulin producing rat tumor cell line, RIN 5AH. Addition of fetal calf serum (FCS) to serum-starved cells in the presence of cycloheximid induced a modest increase in c-fos and c-jun mRNA levels, whereas growth hormone (GH) in the presence of cycloheximid had little or no effect, when added to RIN 5AH cells maintained in 0.5% FCS for 2 days prior to stimulation. Activation of protein kinase C by phorbol ester lead to increased c-jun and c-fos mRNA levels, whereas activation of adenylate cyclase by forskolin increased c-fos mRNA levels. These results suggest that the effects of GH on insulin producing cells are not mediated by activation of c-fos and c-jun transcription.