In order to detect signs of oncogene activity and elucidate their possible role in avian ontogeny we implemented two different strategies. One was to detect either the protein product or messenger RNA in situ at various stages of development. The other was to try and disturb development with retroviruses carrying one or several oncogenes in their activated forms. Time- and tissue-specific expression of c-myc was apparently not related to particular phases of cell evolution, such as population amplification. Rather the presence of c-myc immunoreactive product at particular stages appeared to depend on cell types. c-myb and c-ets messenger RNAs were found expressed preferentially in the blood system, respectively in hemopoietic and differentiating endothelial cells. The developing embryo heart was found to be uniquely sensitive to the effect of retroviruses provided that two conditions were respected. The first was the injection of the virus or construct prior to E3.5. The second was the presence of the v-myc gene, whether alone or associated with one or several other v-onc. In such cases a large proportion (70%) of chick and all quail embryos developed multiple heart rhabdomyosarcomas within 10 days. In chickens the association of a second v-onc or of two others induced the formation of secondary tumors, whose type was determined by the nature of the other oncogene(s).(ABSTRACT TRUNCATED AT 250 WORDS)