Conscious rabbits with previously implanted perivascular balloons for altering blood pressure were studied before and 6 days after: (i) sham-operation (n = 6); (ii) section of carotid sinus nerves alone (n = 7); (iii) section of aortic nerves alone (n = 5); and (iv) combined sino-aortic denervation (n = 5). Sigmoid mean arterial pressure (MAP)-heart period (HP) baroreflex curves were derived and were characterized by: (i) the HP range (HPR) between upper and lower plateau levels; (ii) the median blood pressure (BP50); and (iii) the average gain (G). Resting MAP and HP and the 3 baroreflex parameters were not altered significantly 6 days after sham-operation. After section of the carotid sinus nerves alone resting MAP and BP50 rose by 18 +/- 2.6 mm Hg, resting HP fell by 31 +/- 4.2 msec, whilst HPR and G fell to about 45% and 20 % respectively of initial preoperative control. After cutting the aortic nerves alone there was a similar fall in resting HP as after carotid denervation. However, the rises in resting MAP and BP50 were only about half of those observed after carotid denervation (P < 0.05) and the changes in HPR and G to 65% and 58% of control respectively were also less marked (P ¿ 0.01). The aortic baroreceptor zone also exerted smaller effects on the sympathetic efferent component of the baroreflex HP response. Despite the different effects exerted by each zone on the reflex parameters there was little difference in threshold pressure for producing cardiac slowing. After combined sino-aortic denervation there was a similar rise in resting MAP (19 mm Hg) as after carotid sinus denervation alone, but there was more marked tachycardia, with a fall in resting HP of 82 msec (P < 0.001) and both resting and baroreflex-mediated vagal tones were completely abolished. The small residual pressure-determined HP changes of about 8 msec were entirely sympathetic and probably mediated through cardiopulmonary baroreceptors.