One hundred drug abusers, free of clinical signs or symptoms of disease, were examined by electrodiagnostic techniques. Sensory conduction of median, ulnar and sural nerves was evaluated in terms of latency, velocity and amplitude of evoked potential. Motor nerve latencies and conduction velocities of median, ulnar and personeal nerves were determined. Definite changes in the evoked sensory potentials of median and sural nerves of subjects using heroin or LSD were demonstrated. The sensory amplitude changes were suggestive of axonal degreneration because of normal duration. Maximum motor conduction velocity was abnormal in one patient who admitted using a variety of drugs; five heroin and two barbiturate users showed dispersed motor responses suggesting small fiber involvement. No abnormality could be shown in marjuana smokers. Signifcance of these findings is explained, emphasizing important potential for recognition of subclinical abnormalities and the opportunity for disease prevention.