To investigate a relation between circadian blood pressure and heart rate variation, intra-arterial blood pressure (BP) and heart rate (HR) were recorded during 24 hours in 53 untreated essential hypertensives (EH), 8 secondary hypertensives, 10 normotensives (NT), and 3 patients with Shy-Drager syndrome. Values of systolic BP(SBP) and HR were sampled at about 10 second intervals throughout the 24-hour to calculate the coefficient of correlation between SBP and HR (r_SBP-HR). A significant positive correlation was found between SBP and HR levels in each subject of EH was WHO stage I and II, along with NT (average r_SBP-HR=0.59, 0.40, and 0.54 respectively, p<0.001). Low coefficients of correlation were found in the EH with WHO stage III (r=0.16) and the patients with pheochromocytoma (r=0.05). In contrast, a significant negative correlation was found in the patients with Shy-Drager syndrome (r=-0.44, p<0.001). Since HR is controlled mainly by the autonomic nervous system (ANS), the results suggest that the circadian variation of SBP is also mainly controlled by the ANS in the subjects with high r_SBP-HR and that of SBP controlled by the other factors in subjects with low r_&llt;SBP-HR>.