Twelve temperature-sensitive male patients with multiple sclerosis and 5 normal men were monitored before, during, and after the intravenous injection of 7 to 35 mg of 4-aminopyridine (4-AP) in 1- to 5-mg doses, every 10 to 60 minutes. Static quantitative perimetry, flicker-fusion frequency, visual acuity, and videotaped neurological examinations were performed. Ten of the 12 patients showed mild to marked improvement. Vision improved in 7 patients, oculomotor function in 5, and motor function (power, coordination, gait) in 5. Improvements developed gradually within minutes of drug injection at doses as low as 2 mg, and gradually reversed around 2 to 4 hours after the peak drug effect. No effects were observed in 5 patients given saline injections. No serious side effects occurred in either the normal subjects or the patients receiving 4-AP. It is concluded that 4-AP lessens multiple neurological deficits in multiple sclerosis and, furthermore, that the K+ channel is functional in demyelinated central nervous system axons in humans. The improvements with 4-AP are substantial enough to be of transient therapeutic benefit in selected patients.