The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor with a basic region/helix-loop-helix (bHLH) motif. AhR has been sequenced and the functional domains defined and there is information on the formation of complexes with other peptides and interactions with DNA, although these areas continue to be investigated. AhR mediates many biological effects such as developmental toxicity, including induction of cleft palate and hydronephrosis. This regulatory protein is expressed in embryonic liver and has been immunohistochemically localized in cells of human and mouse secondary palate. The expression of AhR in embryonic tissues and its ability to disrupt development suggests a significant role for this protein in development. The present study examines the pattern of AhR expression in the C57BL/6N mouse embryo from gestation days (GD) 10-16, using in situ hybridization and immunohistochemical analysis. AhR mRNA was localized with 35S-RNA antisense riboprobe (cAh1 probe, 1.8 Kb amino terminal DNA). AhR protein was localized with purified monoclonal antibody (RPT-9) raised against the N-terminal peptide sequence. AhR mRNA and protein were expressed in GD 10-13 neuroepithelium, and as development progressed the levels in brain decreased. GD 10-12 embryos also showed AhR in branchial arches, heart, somites, and liver. AhR protein and mRNA in heart were highest at GD 10-11 and decreased with age. In liver, AhR mRNA and protein levels increased and nuclear localization became more pronounced with gestational age. In GD 14-16 embryos levels in liver and adrenal were highest, but AhR was present in ectoderm, bone, and muscle. AhR expression was specific for both cell type, organ/tissue, and developmental stage, suggesting that this novel ligand-activated transcriptional regulator may be important in normal embryonic development.