The fate of cell-bound [125I]iodo-human GH ([125I]iodo-hGH) was studied in monolayer cultures of hepatocytes from pregnant and nonpregnant rats. Both the binding of [125I]iodo-hGH and its degradation were significantly higher in cells from pregnant than from nonpregnant rats. The positive correlation between the number of binding sites for hGH and the amount of degradation of [125I]iodo-hGH suggests that degradation, at least in part, is a receptor-mediated process. Degradation as a time- and temperature-dependent process was impaired by lysosomotropic compounds such as chloroquine and NH4Cl in a dose-dependent manner. Dinitrophenol, N-ethyl-maleimide, and NaN3 were also effective in preventing degradation, suggesting that an energy-requiring process is involved in degradation of [125I]iodo-hGH. Cell-bound [125I]iodo-hGH became less dissociable as a function of time of association. Degradation of [125I]iodo-hGH accelerated the dissociation of cell-bound radioactivity. Gel-filtration experiments revealed that [125I]iodo-hGH is degraded to smaller molecular species, but only a small portion of the degradation products exists within the cells. These observations suggest that receptor-bound [125I]iodo-hGH is degraded by an internalization process; lysosomal enzymes are probably responsible for the degradation of [125I]iodo-hGH, and the degraded products are rapidly released into the extracellular space.