Piperoxan (50-100 micrograms/kg) injected into the cisterna magna of anaesthetized dogs increased blood pressure, heart rate and resting rate of sympathetic nerve discharge. The baroreflex curve was shifted to the right with an elevation of the upper plateau (when blood pressure was lowered below resting values, renal sympathetic nerve activity rose to an upper plateau) and with no change in baroreflex sensitivity. Catecholamine depletion, produced by reserpine and alpha-methyl-p-tyrosine, did not change the effects of piperoxan on resting sympathetic nerve activity and on the baroreflex curve. Piperoxan (50 micrograms/kg i.c.) enhanced the rate of renal sympathetic nerve discharge in baroreceptor-denervated dogs. Piperoxan (5 micrograms) injected bilaterally into the nucleus tractus solitarii (NTS) increased blood pressure and resting sympathetic nerve activity with a shift of the sympathetic baroreflex curve to the right and no change in baroreflex sensitivity. Piperoxan (5 micrograms) injected bilaterally into the ventrolateral pressor area did not change blood pressure or the rate of sympathetic discharge. However, the baroreflex curve was shifted to the right with an elevation of the upper plateau level and the baroreflex sensitivity was increased. The present findings suggest that tonically active alpha 2-adrenergic mechanisms located within the NTS regulate resting and reflex sympathetic activity. An alpha 2-adrenergic mechanism in the rostral ventrolateral medulla modulates reflex sympathetic activity but has no influence on resting activity.