The level of mRNAs for the c-myc protooncogene and the serum proteins alpha-fetoprotein (AFP) and albumin in liver, visceral yolk sac and gut between day 9 and day 19 of mouse gestation was studied by in situ hybridization employing single-stranded RNA probes. In the prehepatocyte population, c-myc was coexpressed with albumin and AFP. No heterogeneity was noted within this cell population with respect to the expression of these mRNAs up to day 15. AFP expression was high in the liver primordium and rose further until day 15. Albumin mRNA was expressed weakly but distinctly in the hepatic bud and increased throughout fetal life. C-myc expression in prehepatocytes exhibited a maximum around day 13 and a dramatic decline after day 15, but was much lower in other cell types of the fetal liver. In the visceral yolk sac, AFP was strongly expressed, with albumin expression first becoming detectable at day 13, while c-myc mRNA was detected up to day 9. In the endodermal gut epithelium, c-myc expression was high, albumin mRNA was not detected and AFP message was restricted to individual loops of the gut. These results suggest that a period of high c-myc expression in the developing liver may allow rapid expansion of the prehepatocyte population at a specific stage of differentiation.