Nanomolar concentrations of cytosine arabinoside (ara-C), a structural analogue of 2'-deoxycytidine (2'dC) used in the chemotherapy of cancer, proved to be highly effective in preventing the death of postmitotic dopaminergic neurons that occurs spontaneously by apoptosis in mesencephalic cultures. The rescued cells were totally functional and highly differentiated. The trophic/neuroprotective effects of ara-C were (1) specific for dopaminergic neurons; (2) long-lived, remaining detectable several days after withdrawal of the nucleoside analogue from the culture medium; (3) still observed when the treatment was delayed after plating; (4) abolished by an excess of 2'dC or dCTP, or by exposure to the neurotoxin 1-methyl-4-phenylpyridinium; and (5) mimicked by ara-CTP, 5-fluoro-2'-deoxyuridine, and aphidicolin. Autoradiographic studies revealed that ara-C was incorporated exclusively into astrocyte nuclei, suggesting that the dopaminotrophic activity was indirect and resulted from the antiproliferative action of the modified nucleoside on glial cells at concentrations that were not neurotoxic. No evidence was found for putative deleterious or trophic molecules secreted by proliferating or ara-C-treated astrocytes, respectively, suggesting that neuroglial contact may play a role. Our results suggest a possible mechanism underlying neurodegeneration in Parkinson's disease, where selective loss of dopaminergic neurons in the mesencephalon is accompanied by astrogliosis.