The endocytosis of [125I]nerve growth factor (NGF) by cultured rat pheochromocytoma cells, the PC12 line, was studied by ultrastructural quantitative autoradiography. Cells previously grown in the absence of NGF were incubated at 37 degrees C with [125I]NGF for periods of up to 26 h. Under these conditions, PC12 cells have not yet shown outgrowth of neurites. Heavy labeling of the plasma membrane was observed at 2 h. At 6 and 26 h, lower but significant levels of labeling of the plasma membrane were still noted. After 2, 6 and 26 h, endocytosis of [125I]NGF was detected. Low breakdown of [125I]NGF was observed only after 26 h. At 26 h, grain density distributions of [125I]NGF showed significant labeling of lysosomes, while nuclei and rough endoplasmic reticulum showed the lowest levels of labeling. Significant apparent labeling of vesicles of smooth endoplasmic reticulum and of various cytoplasmic components, including cytoskeletal elements, was also observed. These findings indicate that [125I]NGF undergoes endocytosis quite slowly. During the initial phase of the interaction between NGF and PC12 cells, plasma membrane moieties are constantly labeled while lysosomes show progressively increasing uptake of NGF. The pathway of endocytosis of [125I]NGF included vesicles of the smooth endoplasmic reticulum but the Golgi apparatus was not unequivocally labeled. The findings do not support the hypothesis that NGF is transported to the nucleus for the initiation of transcriptional events. Our morphologic observations are consistent with the hypothesis that NGF constantly occupies sites on or adjacent to the plasma membrane, and that it slowly undergoes endocytosis into lysosomes.