The phrenic nerve response to identical carotid sinus nerve (CSN) stimulations was studied in anesthetized, paralyzed and vagotomized cats during apnea induced by hypocapnia. Apnea could be terminated and rhythmic phrenic activity reestablished by CSN stimulation. Magnitudes and latencies of the phrenic response were used as indices of the level of subthreshold respiratory neural activity before stimulation. Increasing hypocapnia progressively reduced the apparent magnitude of the response to a constant CSN stimulus and increased the latency. When the chemical stimulus (PCO2) was held constant at levels subthreshold for rhythmic respiration, it was found that: (1) physical calf muscle stimulation shortened the latency, (2) restimulation of the CSN led to greater magnitudes of response and shorter latencies as the time after a prior stimulation became shorter. The findings support a conclusion that even in the absence of rhythmic respiration there exists a subthreshold respiratory drive which is graded by the level of PCO2 and which is affected by neural input from limb muscles and by the mechanism that causes respiratory afterdischarge.