To evaluate the effect of the prostaglandin inhibitor acetylsalicylic acid (ASA) on rat exocrine pancreas secretion, three groups of rats were administered ASA by infusion: Groups 1-3, 50, 100, and 200 mg/kg body wt, respectively; Group 4 received saline. Twenty minutes later these ASA-pretreated groups were given intraarterial secretin (18 CU/kg) and cholecystokinin (CCK) (18 micrograms/kg). In an additional three groups of seven rats each, saline solution rather than secretin-CCK was given after ASA pretreatment. Pancreatic juice was collected every 10 min by means of a chronic pancreatic fistula. Bicarbonate and protein concentrations were measured and variations in outputs observed. No significant variations were found in the bicarbonate concentrations and outputs of rats with different types of pharmacological treatment, while protein concentrations and outputs were found to vary with time and type of experiment. There was, however, no interaction between these two variables. At lower ASA dosages, the bicarbonate and protein concentrations and outputs of secretin-CCK-stimulated rats were higher than the basal values and the levels of rats without hormonal stimulation. At higher dosages, no difference was found between the two groups. In conclusion, ASA seems to interfere with stimulated pancreatic exocrine secretion of proteins, even when its effect on bicarbonate concentration is factored in, and its effect seems to be present at the highest dosages considered in the study. Among the various hypotheses that may explain this phenomenon, an antagonizing effect of ASA on secretin-CCK action should be the first to be considered.