PROTOCOL OUTLINE: This is a randomized, double blind, placebo controlled study. Patients are randomized to receive 3,4-diaminopyridine (DAP) or placebo orally 3 times daily for 5 days, after which treatment is discontinued and patients are observed for at least 24 hours. At the end of the blinded study, patients may then elect to take open label DAP orally 3 times daily for 6 months; those who do so are monitored for clinical effects and side effects for at least 6 months.