Abstract: Five alkaloids have been isolated from the roots of Cyclea barbata (Wall.) Miers (Menispermaceae) grown in Hai-nan island, China. Four of them are identified to be d-tetrandrine (the main alkaloid), homoaromoline, l-curine and d-isochondodendrine. Alkaloid Ⅴ is not yet identified. The test for neuromuscular blocking activity of the dimethiodide derivatives of these alkaloids showed that l-curine 2 CH₃I was more powerful than the other four and belonged to a nondepolarizing type.The neuromuscular blocking activity of dimethyl-l-curinedimethochloride was found to be about 4 times as powerful as l-curine. dimethiodide The water solubility of the former was 250～300 times higher than that of dimethyl-l-curine-dimethiodide.The clinical tests of l-curine dimethiodide and dimethyl-l-curine dimethochloride on 210 and 412 cases respectively have been completed. The effects of the former approximate to that of α-tubocurarine, while for the latter, satisfactory results of all cases were obtained.