The interaction of diazepam with neuromuscular blocking agents was studied in patients and in several animal nerve-muscle preparations. This drug did not alter the recovery slope of blockade produced byd-tubocurarine, decamethonium, or gallamine. In higher doses diazepam produced a decrease in muscle contractions and blocked the actions of exogenously applied nicotine and acetylcholine in the superfused chick biventer cervicis nerve—muscle preparation. In low doses the commercially available drug given intra-arterially reversed the myoneural blockades produced by both depolarizing and nondepolarizing blockers. However, this property was due to the solvent system of the drug.