The expression pattern of the c-myc protooncogene during fetal development of the mouse was studied by RNA in situ hybridization to tissue sections. In day 15.5 post coitum (p.c.) and day 16.5 p.c. embryos, distinct c-myc expression was observed at the site of brown adipose tissue development. High c-myc expression at this site coincides with expression of the NADH-dependent, cytosolic form of glycerol-3-phosphate dehydrogenase (GPDH). Fat deposition in the form of single small droplets occurs as early as day 15.5 p.c. At earlier stages (day 14.5 p.c. and earlier) the mesenchymal precursor cells at this site show neither c-myc nor GPDH expression. During fetal development the brown adipose tissue increases enormously in size. GPDH expression increases concomitantly as does multilocular fat deposition. However, c-myc expression is not detectable in brown adipose tissue later than day 16.5 p.c.