The factors responsible for 24-hour blood pressure and heart rate variabilities have never been clarified; however, studies performed in unanesthetized animals have shown an increase in blood pressure variability after sinoaortic denervation, and a negative relationship has been reported occasionally between blood pressure variability and baroreflex control of heart rate in humans. We have systematically investigated this issue in 82 ambulant hypertensive subjects using 24-hour intraarterial blood pressure recording (Oxford method) in which blood pressure and heart rate variabilities were measured by calculating the standard deviations of the values obtained throughout the 24 hours or during separate daytime and nighttime periods. Baroreflex sensitivity was assessed by the bradycardic or tachycardic responses to intravenous injections of phenylephrine or nitroglycerin and by the blood pressure response to changes in carotid transmural pressure obtained with a neck chamber. The sensitivity of the baroreceptor-heart rate reflex as Bssessed by the vasoactive drug technique showed a negative relationship with 24-hour blood pressure variability as well as with daytime and nighttime blood pressure variabilities measured separately (r = −0.28 to −0.50,p< 0.05). These Variabilities also correlated negatively with the sensitivity of the baroreceptor-blood pressure reflex as assessed by the neck chamber technique. By contrast, baroreflex sensitivity showed a positive correlation with heart rate variabilities (r= 0.32 to 0.47,p< 0.05). The relationship between baroreflex sensitivity and blood pressure and heart rate variabilities was confirmed when the data were analyzed by multiple regression to adjust for blood pressure and age differences among the 82 subjects. These results suggest that 1) arterial baroreflexes exert a buffering influence on the magnitude of daytime and nighttime blood pressure variabilities in humans; 2) these reflexes favor heart rate variability, which may represent one of the means by which baroreflex stabilization of blood pressure is accomplished; and 3) because of the low correlation indices between baroreflex sensitivity and blood pressure and heart rate variabilities, other factors (probably central in nature) are important in determining the size of these variations.