The half relaxation time (T½R) of adductor pollicis muscle contraction after supramaximal ulnar nerve stimulation was measured in normal women and carriers of Duchenne muscular dystrophy. The T½R after a single stimulus did not differ in these two groups. The decrease of T½R after 2-per-second stimulations, however, was significantly greater in the carriers. The percentage of the T½R decrease after 2-per-second stimulations in all of the carriers was below the mean -2 SD value for the controls. These findings suggest that determination of the decrease of the T½R of the adductor pollicis muscle with 2-per-second stimulations of the ulnar nerve may be a reliable method for detecting Duchenne muscular dystrophy carriers.