To investigate the possible role of ras proteins in the differentiation process signaled by nerve growth factor, we have microinjected the proto-oncogenic and oncogenic (T24) forms of the human H-ras protein into living rat pheochromocytoma cells (PC12). PC12 cells, which have the phenotype of replicating chromaffin-like cells under normal growth conditions, respond to nerve growth factor by differentiating into nonreplicating sympathetic neuron-like cells. Microinjection of the ras oncogene protein promoted the morphological differentiation of PC12 cells into neuron-like cells. In contrast, microinjection of similar amounts of the proto-oncogene form of the ras protein had no apparent effect on PC12 cells. The induction of morphological differentiation by the ras oncogene protein occurred in the absence of nerve growth factor, was dependent on protein synthesis, and was accompanied by cessation of cell division. Treatment of PC12 cells with nerve growth factor or cAMP analogue prior to injection did not alter the phenotypic changes induced by the ras oncogene protein.