The E2F family of transcription factors includes five E2F and three DP forms. E2F is involved in the regulation of cell proliferation, but little is known about E2F function during vertebrate development. We have explored the regulation of E2F expression during mouse organogenesis by in situ hybridization. We find selective up-regulation of E2F-2, E2F-4, and E2F-5 transcripts in epidermis and intestinal epithelium at important developmental stages. E2F-4 transcript levels are high in early, undifferentiated single-cell-layer ectoderm, and later in 13.5-14.5-day-postcoitus (dpc) embryo epithelium, which contains several layers of proliferating cells. E2F-2 is up-regulated following the onset of E2F-4 expression and is first apparent in undifferentiated epithelium at 13.5-14.5 days of gestation. In contrast, E2F-5 transcripts are detected later in gestation, once the epidermis shows evidence of stratification. Stratification of the epidermis into basal, proliferating cells and suprabasal, terminally differentiating cells at 15.5-19.5 days of gestation coincides with expression of E2F-2 and E2F-4 in basal cells and of E2F-5 in suprabasal cells. Similarly, in intestinal epithelium, E2F-4 up-regulation in pseudostratified epithelium at 13.5 days of gestation precedes appearance of E2F-2 transcripts, in 14.5-dpc embryos, in the proliferating, intervillus epithelium. In 16.5-19.5-dpc embryos, no E2F-2 transcripts were detected at the tip of the developing villi, which contain terminally differentiating cells. In contrast, E2F-5 transcripts were limited to the upper half of the villi and were absent in the intervillus epithelium. This suggests that E2F-2 and E2F-4 may participate in maintaining epithelial cells in a proliferative, undifferentiated phenotype, whereas E2F-5 may be important to maintain the differentiated state. Thus, selective regulation of E2F forms occurs during murine epithelial development, irrespective of the ectodermal or endodermal origin of such epithelia.