A new experimental system with cultured gastric cells was developed to estimate the cytoprotective and damaging effects of drugs. The gastric cells were incubated in vitro in medium of pH 3, and their resistance to acid was determined by measuring percentage of damaged cells as a function of time. Pretreatment with 0.5 mM acetylsalicylic acid (ASA) for 24 h decreased their resistance (26%), whereas pretreatment with 0.3-3 micrograms/ml of 16,16-dimethylprostaglandin E2 (dimethyl PGE2) for 4 h increased their resistance (13-16%). The adenosine 3':5'-cyclic monophosphate (cyclic AMP) level in the cultured gastric cells was decreased by 0.5 mM ASA (30%) and increased by 0.3 microgram/ml of dimethyl PGE2 (38%) on treatment for 24 h. These changes reflect phenomena observed in the gastric mucosa, suggesting that this experimental system is valid as an in vitro model. Concentrations of 0.1 mM cyclic AMP and 0.1 mM N6, O2'-dibutyryladenosine 3':5'-cyclic monophosphate (dibutyryl cyclic AMP) were found to exert cytoprotective effects on the cells (5 and 7% increase, respectively). Furthermore, 0.1 mM cyclic AMP caused partial recovery from 0.5 mM ASA-induced decrease in resistance (ASA: 17%, ASA + cyclic AMP: 9%). These findings suggest that cyclic AMP plays a role in protection of cultured gastric cells and presumably also of gastric mucosal cells in vivo.