The response of pancreatic protease synthesis to fish meal-enriched diets was investigated in the pig, which is generally held to be a suitable model for human digestive physiology. In three sets of experiments, pigs were fed either 7, 17, 48, or 68% protein diets for 8 days (1st set), or 17 or 48% protein diets for 3, 6, or 7 days (2nd set), or 7 or 48% protein diets for 4 days (3rd set). At the end of each experiment, the pancreata were removed for biochemical and gene expression assays. The specific activities of amylase, lipase, chymotrypsin, trypsin, and elastase decreased as the result of the 7% protein diet after an 8-day feeding period (lst set). In the same experiment, all the enzyme-specific activities (those of amylase, lipase, chymotrypsin, elastase as well as carboxypeptidases A and B) increased in response to the 48% protein diet, the most strongly affected enzyme being chymotrypsin. Only chymotrypsin and carboxypeptidase A-specific activities were further increased after feeding with the 68% protein diet for 8 days as compared with the 48% protein diet. The amylase, lipase, and trypsinogen mRNA levels remained unchanged throughout the experiments, but the mRNA encoding procarboxypeptidase A2 decreased, and that co&ng for chymotrypsinogen was enhanced after the animals had been fed the experimental diets for 3 days, but showed no change thereafter. Procarboxypeptidase B mRNA increased slightly only after a 6-day feeding period. When pigs were fed the 7 and 48% protein diets for 4 days, the enzymes synthesized in vitro in pancreatic lobules were correlated with the relative levels of the corresponding mRNAs, as measured by means of an in vitro cell-free reticulocyte-lysate translation system: both amylase and carboxypeptidases specific activities and their mRNA levels decreased slightly, while those of serine proteases increased. It was concluded that the biosynthesis of each serine protease was regulated separately and transiently at the pre-translational level. On the other hand, it seems very likely that amylase may be regulated at the translational level, while a multiple-level control process may take place in the case of procarboxypeptidase A2.