The distribution of c-myc and N-myc transcripts during mouse organogenesis was investigated by in situ hybridization and compared to proliferation in several tissues. Only c-myc expression was found during the formation of cartilage, brown adipose tissue, glandula submandibularis, thymus and liver. There was a temporally and spatially ordered expression of N-myc only during the organogenesis of brain, retina and eye lens. In some organs (e.g., in lung and tooth bud), c-myc and N-myc were expressed in a striking complementary pattern that reflected the ontogenic origins of different tissue components. Transcripts of both genes were found in the early gut epithelium, but as formation of villi began, the spatial expression pattern of N-myc and c-myc diverged. The results suggest a link between the proliferative state of cell types and the differential expression of N-myc vs. c-myc. Specifically, c-myc is only expressed in rapidly proliferating tissues, while N-myc expression often persists through cytodifferentiation, e.g., during development of eye lens, retina, telencephalon and gut epithelium. Thus, in spite of the structural similarities of N-myc and c-myc genes and proteins their developmental expression patterns suggest different functional roles.