Six different lines of transgenic mice bearing rat elastase I genes stably inherit both the high-level pancreatic expression and low-level nonpancreatic expression characteristic of each original founding mouse. The high pancreatic expression of the introduced rat genes is transcriptionally determined. In response to high rat elastase I mRNA content in the pancreas, rat elastase I protein is synthesized and secreted at high levels. The stable inheritance of expression of the foreign rat elastase I genes in transgenic mice allows the development of animal lines that synthesize and secrete high levels of foreign protein by the pancreas.