The ultrastructure of perisinusoidal (PS) cells was studied in the liver of young pigs 1-28 days after birth. In immature animals these cells have long cytoplasmic processes often surrounding almost the entire sinusoid. In their close vicinity newly formed collagen filaments are found. While in the adult pig the PS cell cytoplasm is almost totally occupied by a large lipid droplet, in the young animal it contained only a few small lipid droplets, accompanied by a well-developed granular endoplasmic reticulum and Golgi-apparatus, microtubules and filaments 5-6 nm in diameter. The latter formed bundles with patchy dark areas. Findings indicate that during early postnatal development the function of PS cells is protein synthesis, perhaps for the production of reticulum fibres rather than fat or vitamin-A storage. In addition, PS cells by containing contractile elements may regulate the width of the sinusoids.