IGF-1 and IGF-2 stimulate dose-dependently DNA synthesis of nonconfluent cultures of rat fat storing cells, a nonparenchymal type of liver cells pathogenetically involved in the generation of liver fibrosis. Maximum stimulation of [3H] thymidine incorporation of about 2.6-fold above control was reached with 100 ng/ml IGF-1 and 500 ng/ml IGF-2, respectively. The DNA synthesis promoting action of both IGF-1 and IGF-2 was most efficiently potentiated by hepatocyte-conditioned medium raising the stimulatory effect up to 21-fold above control cultures. Lysate of hepatocytes (up to 15 micrograms protein/ml) was not effective in potentiating the effect of IGF-1. IGF-1 is bound to free carrier protein(s) present in the medium of hepatocytes, but obviously absent in cell lysate. Three molecular weight fractions in the ranges of 67 kd, 35 kd, and 25 kd could be identified in the medium, which potentiate the growth-promoting effect of IGF-1. Applying Western ligand blot analysis, three molecular size classes of IGF-1 binding proteins in the conditioned media of rat hepatocytes were determined. The major binding protein had a M(r) of 28-34 kd, a minor portion was localized at M(r) 24 kd, whereas trace binding affinities were found at M(r) of about 95 kd. It is suggested that IGF-1, IGF-2 and the complex array of IGF-binding proteins secreted by hepatocytes might be involved in the paracrine regulation of growth of fat storing cells.