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The role played by the cardiac sympathetic fibers in the pulmonary depressor reflex was analyzed in twenty dogs. The selective perfusion with homologous blood of the inferior lobar vessels of the left lung with pressures of 40 to 60 mmHg decreased the spontaneous background discharges recorded from the left superior or inferior cardiac sympathetic nerves. This decrease was maximal at perfusion pressure of 80-100 mmHg. Following the decrease in the sympathetic discharges, the systolic and diastolic systemic arterial blood pressure decrease about 10 per cent. These changes were reversible when the perfusion pressure was returned to the control. The intravenous injection of atropine sulphate did not change either the systemic hypotension or the responses of the sympathetic efferent discharges induced by elevation of the pressure in the vascular bed of the lung lobe. Thus, it is believed that the systemic arterial blood pressure during, this reflex may have fallen due to a diminution of the vascular tone caused by a decrease in the sympathetic efferent discharges. After transection of the vagus nerve ipsilateral to the tested lobe, the reduction of the sympathetic discharges as well as the decrease of the systemic arterial blood pressure were no longer observed. Our results further substantiate the concept that the vagus nerve is the afferent pathway for the pulmonary depressor reflex, and it may be con