Sigmoid renal baroreflex curves relating mean arterial pressure to integrated renal sympathetic nerve activity were obtained in anaesthetized rabbits with previously implanted balloons to raise and lower blood pressure. Propranolol was infused to reach plasma levels averaging 324 ng/ml. This reduced blood pressure by 9.6 +/- 1.1 mm Hg, but had no effect on resting sympathetic discharge. Propranolol lowered the threshold of the renal baroreflex. Median blood pressure was reduced by 15.4 +/- 1.9 mmHg but there was no change in gain or sympathetic activity range. Thus, at a given blood pressure there was diminution of sympathetic discharge compared with control. Similar changes occurred after giving clonidine. However, "non-specific" produced by bleeding or nitroprusside infusion produced no resetting of the baroreflex curves, though the resting sympathetic discharge increased. The effects of propranolol (plasma levels 137 and 348 ng/ml) on arterial baroreceptor discharge were studied by deriving mean arterial pressure-integrated aortic nerve activity curves. Propranolol produced a reduction of aortic nerve discharge of about 7% of control. Single unit analysis showed a small reduction in firing frequency/sec near threshold, which was sufficient to explain the changes in integrated aortic nerve discharge. Since the changes in input from the aortic baroreceptors do not account for the reduction in threshold of the renal baroreflex, we conclude that the latter is due to the central nervous action of propranolol.