Using simultaneous recordings we have made in Man a comparative study of: the sural nerve afferent volley, the nociceptive flexor reflex of a muscle of the lower limb and the associated painful sensation. Two types of stimulations were used, a single short duration electric stimulus, and a train of electric shocks (100/sec). With a single stimulus, the nociceptive flexor reflex and the painful sensation develop only when A delta fibers are recruited. On the other hand, when the stimulations are given by trains the nociceptive flexor reflex and the painful sensation can develop with a stimulus sub-liminar to the threshold of A delta fibers, when A alpha fibers are recruited. When the stimulus activate both A alpha and A delta fibers, the flexion reflex and the pain disappear when a selective blockade of the A delta group is exerted by means of Lidocain.