We compared the effect of high protein diets enriched with casein, fish, or soybean on enzyme content and mRNA levels in pancreata of postweaning and adult mts of two different strains. In the first experiment, 72 male Fischer rats (age 5 weeks) were divided into six groups and fed with one of six diets containing 20% or 50% protein as fish meal, casein, or soybean for 1 or 3 weeks. In a second experiment, 36 Fischer and 36 Wistar rats were divided into six groups and fed with the same diets for I week. In both experiments, rats were sacrificed at the end of the experimental feeding period and pancreata were excised and prepared for biochemical assay and mRNA extraction. Activities and mRNA levels were determined for each enzyme (amylase, lipase, chymotrypsin, trypsin, and elastases). Pancreas weight and its total protein content were modulated by the amount of dietary protein and duration of diet. These parameters were significantly different in Fischer and Wistar rats. In the latter strain, the nature of dietary protein also influenced pancreas weight. In Fischer rats, amylase specific activity was decreased after feeding 50% casein. diet for I or 3 weeks and 50% fish or soybean diets for 3 weeks. The decrease of specific mRNA was more pronounced after a 3-week than After a I-week feeding, suggesting that transcriptional regulation replaced progressively a translational one. In Wistar rats, amylase specific activity was not modified, but mRNAs were decreased after feeding high-protein diets. Lipase specific activity and mRNAs were not modified by any diet in any group. Chymotrypsin specific activity was increased after feeding 50% casein and soybean diets for 1 week and any 50% protein diet for 3 weeks. This effect was more pronounced in adult rats fed high protein diets for I week. In young Fischer rats, chymotrypsinogen mRNAs were increased after feeding 50% casein diets; in adult rats of both strains this parameter was increased by all diets except when the 50% soybean diet was provided to Wistar Trypsin specific activity was increased after feeding 50% casein diet for I week and 50% fish or soybean diet for 3 weeks in Fischer rats bur not altered in Wistar The expression of trypsinogen mRNA was only increased after feeding 50% casein diet to both strains and 50% soybean diet to Wistar rats, suggesting that the regulation of its expression is different with the nature of protein. Elastase specific activities were increased by high-protein diets in both strains, but this effect was more pronounced in Wistar rats; these enzyme mRNAs were not altered, suggesting that the regulation was translational. In conclusion, it appears that the kinetics of adaptation of enzymes is different depending on the nature of dietary protein and the strain of rats used in the experiment Amylase biosynthesis is regulated at the transcriptional level. Chymotrypsinogen and trypsinogen mRNA level showed that casein-induced adaptation was modulated via transcription, while other diets induced adaptation via posttranscriptional events. Elastases adapt differently depending on the nature of the protein and the regulation of their expression is mostly posttranscriptional.