In the cat and monkey the fastest axons in the peripheral nerve are group I afferents from muscle, but there are no definitive data on conduction velocity for these afferents in human subjects. Knowledge of the relative conduction velocities of muscle and cutaneous afferents is important for the interpretation of reflex studies, evoked potentials and other aspects of motor control. To rectify this deficiency, the conduction velocities of the fastest muscle and cutaneous afferents were determined for the median, ulnar and tibial nerves of normal subjects. Low-threshold muscle afferents innervating abductor pollicis brevis, abductor digiti minimi and abductor hallucis were stimulated selectively through a microelectrode inserted percutaneously at the motor point. Low-threshold cutaneous afferents were stimulated with ring electrodes around the proximal phalanx of digits II or V for the upper limb and digit II for the lower limb. Compound action potentials were recorded with bipolar near-nerve electrodes at two sites in the proximal limb segment and conduction velocities of the fastest afferents in the neural volley calculated. The mean conduction velocities of the muscle and cutaneous afferents were, respectively, 74.7 +/- 6.5 m.s-1 and 80.3 +/- 6.7 m.s-1 for the median nerve, 67.5 +/- 10.2 m.s-1 and 67.5 +/- 10.5 m.s-1 for the ulnar nerve, and 54.7 +/- 3.4 m.s-1 and 52.8 +/- 3.2 m.s-1 for the tibial nerve. For upper and lower limb nerves the conduction velocities of low-threshold muscle and cutaneous afferents were not significantly different when measured over the same proximal segment.