The expression of laminin and type IV collagen messenger RNA in rat liver was investigated in relation to liver development. Levels of messenger RNA for laminin chains A, B1 and B2, and α1 and α2 chains of type IV collagen were studied by Northern-blot and dot-blot analysis. Although the expression of messenger RNAs for laminin B1 and B2 chains was higher in 19-day gestational fetal liver when compared with the adult, the highest level was detected in 15-day newborns. This finding coincides with the final establishment of the mature liver. Laminin A messenger RNA was not detected in the 19-day gestational fetus or in the neonatal period. In contrast, α1-(IV) and α2-(IV) collagen messenger RNAs maintained high levels in the early neonate and then decreased gradually after 15 days. Our findings suggest that during late stages of liver ontogeny the main components of liver basement membrane proteins undergo a significant alteration in their synthesis. The data further suggest that laminin may play a role in the final stages of hepatic differentiation.