ets-1 and ets-2 genes have previously been identified by their sequence homology to the v-ets oncogene of the avian erythroblastosis virus, E26. These cellular genes have been shown to function as transcription factors important in lymphoid differentiation and activation and cellular proliferation. In this study, we have broadly analysed the differential expression of ets-1 and ets-2 during murine development using in situ hybridization. Our results indicate that these transcription factors are expressed in multiple tissues during critical stages of embryo formation and organogenesis, suggesting that these genes may serve multiple functions during mouse development. The patterns of expression of both genes are quite different as early as day 8.0 of gestation. ets-1 expression is clearly observed during a narrow developmental stage in the developing nervous system, including the presumptive hindbrain regions, the neural tube, as well as neural crest and the first and second branchial arches, ets-2 expression is limited to the developing limb buds and distal tail. At later times, ets-1 expression is observed in developing vascular structures, including the heart, arteries, capillaries and meninges, whereas ets-2 is highly expressed in developing bone, tooth buds, epithelial layers of the gut, nasal sinus and uterus, and several regions of the developing brain. Both ets-1 and ets-2 are expressed in developing lung, gut and skin. High levels of expression in both genes is observed in adult lymphoid tissues, but in different tissue subsets. ets-1 is expressed in the adult lung, gut mesenchyme and bone marrow. ets-2 continues to be expressed at low levels in several adult tissues, except in the differentiated brain, where substantial levels of expression are found in particular regions of the mature brain. These results demonstrate that ets-1 and ets-2 are differentially regulated, are widely expressed in many tissues during murine embryogenesis and may play important roles in cellular proliferation and differentiation during mouse development.