The cardiovascular effects of bilateral lesions of the nucleus tractus solitarii (NTS) were compared with those of subsequent sinoaortic denervation in the same dogs. Destruction of the lateral but not the medial component of the NTS between + 0.5 and 3 mm anterior to the obex produces mild hypertension and tachycardia, not always sustained for more than 2 weeks. Rises in pressure were accompanied by increased lability which was not present regularly in all dogs but correlated with the baseline level of arterial pressure. On the other hand, sinoaortic denervation following lateral NTS lesions produced the first demonstration of fulminant hypertension in the dog, which led to death within hours. These data suggest that, while NTS lesions in the dog probably only partially interrupt central baroreceptor pathways, the addition of sinoaortic denervation completely disrupts baroreceptor inputs to the central nervous system, thus releasing central sympathetic outflow completely from baroreceptor inhibition.