There is no nerve conduction study for the medial brachial cutaneous nerve in the literature. This study was done in the military hospital # 501 from feb 1st 1997 to june 1st 1999. Three hundred healthy adult (220 male) with a mean age of 40 +/- 13 (range, 14-66) years were studied. At first the sensory nerve action potential of median and ulnar and medial antebrachial cataneous nerves were recorded to show these roots C8-T1 are intact. Then the medial brachial cutaneous nerve was stimulated at the medial border of arm on the line that connects axilla to medial epicondyle (parallel with mid axillary line) there is the juntion site of coracobrachialis muscle to humerus, at the medial border of bicept brachialis muscle, and recording was done on 2 cm above the medial epicondyle. (10 cm under stimulating site). In all of them recording of sensory nerve action potential (SNAP) was done easily and the wave was a biphasic one with the primary negative phase. The latency was 2 +/- 0.3 ms (range, 1.4-2.6) and the amplitude of SNAP was 30 +/- 10 mu v (range, 10-50). The nerve conduction velocity was 61 +/- 4 m/s (range, 53-69). We think with regard of the intensity and site of stimulation and recording area this wave is not due to compound nerve action potential of median or ulnar nerve. This study may be useful in evaluation of T1 root and in the difference of medial cord and lower trunk lesions with the ulnar and medial part of median nerve injury.