Receptor-mediated internalization and degradation of insulin-like growth factors, IGF-I and IGF-II, were studied in primary cultures of neonatal rat astrocytes. Surface-bound IGF-II was rapidly internalized, and 80% of cell-associated radioactivity was located intracellularly after 30 min. IGF-I was internalized at a slower rate, and only 40% of cell-associated radioactivity was inside the cell after 30 min. A pulse-chase experiment demonstrated that 55% and 70% of internalized IGF-I and IGF-II, respectively, was degraded to free amino acids after a 3-hr chase. Lysosomal and protease inhibitors had different effects on the binding, internalization, and processing of IGF-I and IGF-II. Inhibition of lysosomal acidification by chloroquine increased the amounts of surface-bound IGF-II and intracellular IGF-I and reduced the degradation of IGF-I. The chelating agent phenanthroline increased the surface binding of IGF-I and IGF-II and internalization of IGF-II and reduced the degradation of IGF-I and IGF-II. Finally, receptor-bound IGF-II on the cell surface was decreased with increasing cell density, whereas IGF-I binding was unaltered. Our data suggest that cell-surface expression of IGF-I receptors and IGF-II receptors is regulated by different mechanisms and that receptor-bound IGF-I and IGF-II are trafficked and processed by different intracellular pathways in neonatal rat astrocytes.