To determine whether differential sympathetic nerve responses to hypoxia may be explained by opposing effects of hypoxia upon sympathetic premotor neurons. in the rostral ventrolateral medulla (RVLM), the cardiac sympathetic nerve and the renal sympathetic nerve were recorded in anesthetized and vagotomized rabbits. Renal sympathetic nerve was activated by the injection of sodium cyanide solution close to the bifurcation of the common carotid artery and/or inhalation of hypoxic gas (3% oxygen / 97% nitrogen). On the other hands, cardiac sympathetic nerve was inhibited by these stimuli. Barosensitive (inhibited by the stimulation of baroreceptor afferents) reticulospinal (antidromically activated by the stimulation of the spinal cord) neurons in the RVLM were divided into three groups according to their responses to hypoxic stimulation: neurons (Type I, n = 25) whose activity was inhibited by the injection of sodium cyanide solution close to the bifurcation of the common carotid artery and/or inhalation of hypoxic gas, neurons (Type II, n = 99) whose activity was facilitated by the same stimulation, and neurons (Type III, n = 11) whose activity was not changed. These data indicated that the differential responses of cardiac and renal sympathetic nerves are due to opposing effects of hypoxia upon individual RVLM neurons.