P latinum compounds induce apoptosis in malignant cells and are used extensively in the treatment of cancer. Total dose is limited by development of a sensory neuropathy. We now demonstrate that when rats are administered cisplatin (2 mg/kg i.p. for 5 d), primary sensory neurons in the dorsal root ganglion die by apoptosis. This was reproduced by exposure of dorsal root ganglion neurons and PC12 cells to cisplatin (3 microg/ml) in vitro. Apoptosis was confirmed by electron microscopy, DNA laddering, and inhibition by the caspase inhibitor z-VAD.fmk (100 microM). Cell death in vitro was preceded by upregulation of cyclin D1, cdk4, and increased phosphorylation of retinoblastoma protein; all are indicators of cell cycle advancement. The level of p16(INK4a), an endogenous inhibitor of the cyclin D1/cdk4 complex decreased. Exposure of PC12 cells and dorsal root ganglion neurons to increased levels of nerve growth factor (100 ng/ ml) prevented both apoptosis and upregulation of the cell cycle markers. Cancer cells without nerve growth factor receptors (gp140TrkA) were not protected by the neurotrophin. This indicated that cisplatin may kill cancer cells and neurons by a similar mechanism. In postmitotic neurons, this involves an attempt to re-enter the cell cycle resulting in apoptosis which is specifically prevented by nerve growth factor.