Rat glioma C6 cells were employed to determine the vulnerability of the CNS-derived cells to cocaine. The cells were cultured either in the presence of serum or in serum-free (defined) medium to model glial development in the normal brain. In serum-containing medium, cocaine in amounts up to 100 micrograms/ml neither retarded cell proliferation nor altered cell morphology. In the absence of serum, the culture growth was profoundly retarded and cell death was observed with amounts as small as 0.1 micrograms/ml. Even brief (24 hrs) exposure to low cocaine concentrations in serum-free medium irreversibly decreased the cell number. However, an initial 24 hr exposure to 0.1-2.5 micrograms/ml cocaine prolonged survival of cells subsequently exposed to a lethal concentration (100 micrograms/ml). Benzoylecgonine in amounts up to 100 micrograms/ml had no effect on cell proliferation, with or without serum. These data show that cocaine in the absence of serum is highly cytotoxic, which indicates a possibility that the blood-brain-barrier-protected CNS cells may be particularly vulnerable to the drug when it enters the nervous system.