In normal human subjects the twitch force in the adductor pollicis muscle elicited by supramaximal stimulation of the ulnar nerve is significantly lowered by diazepam whereas the tetanic force is not influenced. The evoked potentials recorded electromyographically from the same muscle are also not influenced by the compound. It is suggested that diazepam has an effect on the contractile system of skeletal muscle or on electromechanical coupling.