Regulation of the expression of pancreatic amylase genes was studied by comparing groups of rats fed diets with high (75%), intermediate (20%) and low (11%) carbohydrate content. Animals on the high carbohydrate diet had nine times as much amylase mRNA as those on low carbohydrate diet, and twice as much as the intermediate group, as determined by filter hybridization of equal amounts of total pancreatic RNA to an excess of a cloned rat amylase cDNA probe. Parallel results were obtained when levels of translatable amylase RNA were compared by means of an RNA-dependent rabbit reticulocyte cell-free system. Amylase mRNA-directed synthesis represented 35% of the total in the high carbohydrate group, 4% in the low group and 14% in the intermediate group. Relative rates of amylase synthesis, determined 30 min after [3H]phenylalanine injection, followed the same pattern. While 37% of total was incorporated into amylase in the high carbohydrate group, only 8% was incorporated in the low carbohydrate group, as compared with 22% in the intermediate group. These data indicate that modifications of diet composition alter the expression of pancreatic amylase genes as a consequence of changing the level of their transcript, and that pancreatic amylase production is mostly regulated at the pre-translational level.