The reflex increase in ventilation (Ve) produced by natural stimulation of certain sensory receptors in gastrocnemius muscle was studied in dogs anesthetized with Na pentobarbital. Reflex increase in Ve occurred when the endings of group III fibers and nonmedullated fibers were stimulated (by stretching, pressing, or squeezing the muscle locally), whereas the endings of group I and group II fibers were blocked by repetitive antidromic stimulation. Stimulation of endings of nonmedullated fibers alone also increased Ve after all the medullated fibers were blocked either by cooling the medial gastrocnemius nerve to 5 C or by stimulating lateral gastrocnemius nerve repetitively. It was concluded that there were sensory endings of nonmedullated fibers in muscles which are stimulated by stretching, pressing, or squeezing the muscle locally; these endings produce reflex hyperventilation. antidromic stimulation; gastrocnemius muscle