The effects of steroid hormones on glutamate neurotoxicity were examined in cultured spinal cord neurons. The extent of neuronal damage, produced by glutamate exposure for 15 min, was estimated based on the activity of lactate dehydrogenase released from degenerated neurons to the media during 24 h of post-exposure incubation. This damage was dependent on the glutamate concentrations used. The addition of dexamethasone, a synthetic steroid, in post-exposure media remarkably reduced the extent of damage in a dose-dependent manner. The half effective concentration for the steroid was approximately 0.7 microM, which was in the range of pharmacological concentration. Dexamethasone was effective even when it was added 2 h after glutamate exposure. Some endogenous steroid hormones--aldosterone, progesterone and testosterone--also showed similar neuroprotective effects. However, cholesterol, a precursor of these steroid hormones, had no effect on glutamate neurotoxicity. This direct protective effect on neurons against glutamate neurotoxicity may explain, at least partly, the mechanisms of beneficial effects of steroid hormones on in vivo spinal cord injury.