. Levamisole hydrochloride in low concentration (5 ng/mI) stimulated the movements of the whole worm as well as of the nerve-muscle complex of Setario cervi suspended in an isolated organ bath. At higher concentration (25 ng/mI) the response was biphasic consisting of initial stimulation followed by irreversible paralysis. The initial stimulant effect is not due to release of ACh as it is not blocked by prior addition of d-tubocurarine to the bath fluid. Levamisole in 5 x 25 mg/kg daily dose caused disappearance of microfilariae from the peripheral circulation in 7 of the 8 rats treated: The drug at this dose level had some macrofilaricidal action as well. A higher dose of 5 x 50 mg/kg daily caused complete disappearance of microfilariae from peripheral circulation of all the rats and produced a significant lethal effect on the adult worms The antifilarial efficacy of the drug needs evaluation in domestic animals and men.