In supine chloralose-anesthetized and mechanically ventilated dogs, we assessed the effects of group III and IV thin-fiber phrenic afferents on cardiorespiratory control by injecting capsaicin into the phrenic artery of an in situ isolated and innervated left diaphragm. Inspiratory motor drive was assessed by measuring the electromyogram of left and right diaphragm, left parasternal, and mylohyoid muscles in five protocols. 1) Three boluses (2 ml) of capsaicin (1, 10, and 50 micrograms/ml) were injected 30 min apart. Only the 50-micrograms/ml injection elicited a significant increase in arterial pressure, heart rate, and inspiratory motor drive. 2) Repeated doses of capsaicin were tested. The pressor and hyperpneic responses were weakened. 3) High doses of capsaicin (100 and 500 micrograms/ml) were given. Hyperpneic and pressor responses were similar to those elicited by the 50-micrograms/ml dose. 4) When the left phrenic nerve was sectioned, the pressor and hyperpneic responses to the 50-micrograms/ml injection were abolished. 5) Capsaicin (50 micrograms/ml) was infused into the arterial supply of the in situ vascularly isolated and innervated gastrocnemius. Arterial pressure, breathing frequency, and inspiratory motor drive to all inspiratory muscles increased significantly and to a greater degree than in the diaphragm. In conclusion, diaphragmatic thin-fiber afferents have an excitatory effect on the inspiratory motor drive and arterial pressure that is similar to that seen in limb muscles.