Expression of the homeodomain transcription factor IDX-1 (also known as IPF-1, STF-1, and PDX-1) is required for pancreas development, because disruption of the gene in mice and humans results in pancreatic agenesis. During embryonic development the idx-1 gene is first expressed in a localized region of foregut endoderm from which the duodenum and pancreas later develop. To more fully understand the role of IDX-1 in pancreas development, transgenic mice expressing the Escherichia coli lacZ gene under control of the 5′-proximal 4.6 kb of the idx-1 promoter were created as a reporter for the developmental expression of IDX-1. Here we show that the determinants for the developmental and tissue-specific expression of the endogenous idx-1 gene are faithfully reproduced by the 4.6-kb region of the idx-1 promoter. Expression of lacZ is detected in the development of the exocrine and endocrine pancreas in pancreatic ducts, common bile and cystic ducts, pyloric glands of the distal stomach, Brunner’s glands, the intestinal epithelium of the duodenum, and the spleen. The observed spatial and temporal pattern of lacZ expression directed by the IDX-1 promoter further supports an important role of IDX-1 in specifying the development of several endodermal structures within the midsegment of the body. An unexpected finding is that IDX-1 promoter-driven (transcriptional) lacZ activity does not always coincide with the localization of IDX-1 messenger RNA by in situ hybridization and IDX-1 protein by immunocytochemistry in adult rat duodenum, suggesting the existence of regulation of IDX-1 expression at the posttranscriptional level of expression of the idx-1 gene.