Nitric oxide (NO) is formed by different cell types in the pancreas. In this study, inhibition of endogenous nitric oxide by N(omega)-nitro-L-arginine (L-NNA) reduced the urinary excretion of NO(2)/NO(3) and raised serum L-arginine and the NO donator S-nitroso-N-acetylpenicillamine (SNAP) increased the urinary excretion of NO(2)/NO(3). The peptide cholecystokinin-8 (CCK-8) has a strong influence on exocrine pancreatic proliferation. Rat pancreas was excised and studied with regard to tissue weight, protein and DNA contents after 3 days of treatment with saline, L-NNA or SNAP given separately or combined with CCK-8. Further, proliferation of different pancreatic cells was studied with [3H]-thymidine incorporation and apoptotic activity was studied by analysing caspase-3 activity and histone-associated DNA fragments. The effects of L-NNA indicate that endogenous nitric oxide formation has a tonic inhibition on apoptosis in the pancreas during both basal condition and growth stimulation by CCK-8. In CCK-induced hyperplasia, NO inhibits the proliferation of acinar cells but stimulates ductal cells. Endogenous NO may regulate the balance between proliferation and apoptosis and in a situation of growth stimulation by CCK-8, it has a tonic inhibition on both mitogenesis and apoptosis thus slowing down the acinar cell turnover in the pancreas.