Sino-aortic denervation (SAD) in rats alters completely the pattern of pressure changes during sleep: from unchanged to a rise in SS and from slight increase to a market drop in DS. Rats with mild renal hypertension (1K - 1C) behaved like normotensive rats, whereas in those rats with hypertension accompanied by overactivity of RAS, the pattern of MAP changes during sleep was similar to SAD rats. Since acute SAD also produced overactivity of RAS we studied SAD rats treated with Captopril or when RAS was normal at the chronic phase of SAD and we showed that impairment of baroreceptor function per se determines the typical alteration of BP during sleep. Intracerebral infusion of angiotensin modifies the pattern of pressure changes during sleep in a way similar to that produced by impaired baroreceptor function (SAD), probably by altering the central integration of the baroreceptor reflexes. Therefore the pattern of pressure changes during sleep seems to be a sensitive index of the functional integrity of the baroreceptor reflex.