In unanaesthetized cats the most striking effects of sino-aortic denervation (SAD) consist of a marked increase in blood pressure variability and a concomitant marked reduction in heart rate variability. Because the relative contribution of carotid and aortic baroreceptors to these phenomena has never been assessed, blood pressure (intra-arterial catheter) and heart rate were measured in unanaesthetized, unrestrained cats for 8-10 h under three conditions: (1) intact animals, (2) 1 week after section of the carotid sinus or the aortic nerves and (3) 1 week after SAD. Blood pressure and heart rate signals were analysed by a computer to provide mean values and variation coefficients of variation i.e. blood pressure and heart rate variabilities, for each recording period. In the intact cats the coefficient of variation was 6.6 + 0.6% (mean ± s.e.) for mean blood pressure and 11.2 ± 1.7% for heart rate. The coefficient of variation for mean blood pressure was not altered by either the aortic or the carotid sinus nerve section, a marked increase being observed only after SAD (11.5 ± 1.3%, P < 0.01). On the other hand, the coefficient of variation for heart rate was reduced either by the carotid or by the aortic nerve section. The reduction observed following the carotid baroreceptor denervation accounted for the greater fraction of the overall reduction (74 versus 26%) in coefficient of variation for heart rate observed after SAD (4.8 ±0.9%, P< 0.01). These data show that (1) the carotid sinus and aortic nerves are similarly involved in control of blood pressure variability, (2) the carotid sinus predominates over the aortic nerves in control of heart rate variability and (3) whereas either the carotid or the aortic baroreceptor area provides a full reflex buffering action on blood pressure variability (whose control mechanisms are redundant), this is not the case for heart rate variability. This control depends on integrity of both reflexogenic areas and, in particular, on the carotid baroreceptors.