he intracellular distribution of 125I-labeled nerve growth factor (NGF) in rat pheochromocytoma PC12 cells was studied by quantitative electron microscopic (EM) autoradiography and by subcellular fractionation. PC12 cells were grown as monolayer cultures in medium supplemented with serum in the presence of 125I-NGF. EM autoradiography showed that 125I-NGF was localized at the plasma membrane and cytoplasmic compartments but did not accumulate in the nuclear chromatin or in the nuclear membrane compartment of cells analyzed after 1 hr and 1, 2, and 8 d of incubation with 125I-NGF. 125I-NGF also was not detected in nuclear subcellular fractions prepared from cells grown in serum-supplemented medium either in suspension for 1 d or in monolayer cultures for 1 to 8 d. In contrast, and in confirmation of the results of Yankner and Shooter (Yankner, B. A., and E. M. Shooter (1979) Pro. Natl. Acad. Sci. U. S. A. 76: 1269–1273), about 60% of the cell-bound 125I-NGF was found in the nuclear pellet after cell fractionation if the cells had been kept previously in suspension for 1 d in phosphate-buffered saline supplemented with 0.2% glucose, 0.1% bovine serum albumin, and 125I-NGF. The ultrastructure of PC12 cells grown under such conditions, however, revealed signs of varying degrees of damage. Autoradiography of the nuclear pellet from these cells showed the grains to be located mainly over damaged nuclei or over cell debris between nuclei. It is concluded that NGF, after binding to specific receptors at the plasma membrane, is transferred to membrane- confined cytoplasmic compartments but does not have to be transferred further to the nuclear membrane or to the nuclear chromatin as a prerequisite for its physiological action.