myc and N-myc are related genes whose similar protein products may be used for different purposes in vertebrate organisms. We have explored this possibility by using hybridization in situ to examine the expression of myc and N-myc during gastrulation of mouse embryos. Throughout gastrulation, myc RNA was most abundant in extraembryonic cells; by contrast, N-myc RNA was found at highest levels in the expanding primitive streak and other portions of the embryonic mesoderm. Differentiation of mesoderm to epithelioid cells was accompanied by diminished expression of N-myc. Expression of myc was not an inevitable correlate of cellular proliferation. Instead, the gene appeared to be regulated in concert with changes that affect a diversity of cellular properties, including proliferation, invasiveness, and differentiation.