The pattern of conduction times in the distribution of the radial nerve was studied in 24 normal subjects and 12 patients with radial nerve lesions. The radial nerve was stimulated supraclaviculary, in the midarm, and immediately above the lateral epicondyle of the humerus. Recordings were made from muscles in the distribution of tactically placed radial nerve branches. The normal range of motor nerve conduction velocity is reported to the brachioradialis, extensor digitorum communis, and anconeus muscles. The normal range of distal latency values is also reported to these muscles and to recordings made from the triceps muscle at progressive distances from the supraclavicular stimulating point. In determining the conduction velocity of the radial nerve, it was especially important to measure potentials of similar appearance elicited by stimulation of the nerve at different levels. There was considerable interference by potentials generated in adjacent muscles and conducted in volume to the recording electrode. This was most pronounced in cases with nerve lesions when high amplification was used in recording. Measurement of the distance was made with obstetric calipers, which gave a reliable and valid value. The mean conduction velocity in the branches of the radial nerve to the brachioradialis and extensor digitorum communis muscles was higher than that to the anconeus muscle. However, all of these conduction velocities are higher than the values reported in the nerves to the distal muscles of the extremity. The mean latency of the response to the triceps muscle on supraclavicular stimulation increased with increasing distance from the point of stimulation. This latency increase was related to the anatomy of nerve supply and end plate organization in the muscle. The clinical findings and results of electrical stimulation of the radial nerve in patients with sleep palsy closely paralleled those in patients with pressure palsy of the radial nerve. These latter were patients in whom the site of the lesion was clearly marked at the lateral border of the humerus where the radial nerve pierces the lateral intermuscular septum. It supports this site as the locus of the lesion in sleep palsy. It was sometimes found, in cases with pressure palsy, that the nerve could be excited proximal to the lesion, even in the presence of a complete paralysis, indicating a discrepancy between the nerve''s capacity for the conduction of voluntary or reflexly inaugurated impulses and electrically excited impulses. The examination was also found to be of aid in the localization and prognosis of traumatic lesions of the radial nerve.