Nine neurologically intact subjects were studied to demonstrate the feasibility of stimulating the muscles of expiration during, and in synchrony with, naturally occurring breathing. A breath-by-breath analysis showed that both tidal volume and the frequency of respiration could be increased during periods of electrical stimulation. A single subject with complete spinal cord injury was studied to eliminate the possibility that the results from the normal subjects could be attributed entirely to either subconscious or conscious volitional response to the stimulation. The results provide a basis for future studies with patients in borderline ventilatory failure.