The protective effect of chlorpromazine on rat spinal cord injury was investigated using a dynamic impact model. A 10 g weight was dropped 5 cm on an impounder placed on the exposed spinal cord at the T-11 level. Changes in potassium concentration on the epidural surface of the injured spinal cord were measured using a combined impounder-K+ electrode assembly. Recovery of motor performance was estimated using the modified Tarlov score. In the injury control (no treatment) group, the recovery was slow. Animals were still paralyzed 4 weeks after injury and none of them could walk; the Tarlov score was 1.88 +/- 0.78 (S.D.). In contrast, the chlorpromazine-treated group (20 mg/kg i.p. 30 min prior to injury) recovered significantly in 4 weeks. Animals could either support body weight or walk with some deficit; the Tarlov score was 4.0 +/- 0.35. Chlorpromazine inhibited potassium efflux from the spinal cord after contusion. Possible mechanisms of protection of neural cells by chlorpromazine are discussed.