Abstract-Ultradian rhythms in blood pressure (BP) are known to exist, but their modification in hypertension is largely unknown. The present study was undertaken to assess the integrity of ultradian and 24-hour BP rhythms in dipper (nϭ100) and nondipper (nϭ20) hypertensive patients compared with 44 dipper normotensive individuals. Fourier analysis was used to fit ultradian (12, 8, and 6 hour) and 24-hour rhythms in BP and heart rate (HR). Mesor, amplitude, and acrophase were calculated for individual and overall rhythm curves. All subjects showed significant ultradian or 24-hour BP and HR rhythms. Systolic and diastolic BP mesor was higher in hypertensive patients compared with normotensive patients. The percentage of variability in ambulatory BP that could be explained by fitting ultradian and 24-hour rhythms was reduced in nondippers compared with normotensives or dippers. Amplitude of ultradian and 24-hour rhythms in BP increased in dippers and decreased in nondippers. Ultradian and 24-hour rhythms in HR did not differ among the 3 groups examined. Results indicate that in nondippers, blunted ultradian and 24-hour rhythm amplitude in BP was accompanied by a loss of rhythm integrity. Key Words: blood pressure monitoring, ambulatory Ⅲ human Ⅲ hypertension, arterial Ⅲ circadian rhythm C ircadian rhythm in blood pressure (BP) has been recognized as a prognosis marker in hypertensive patients. 1 In normotensives as well as in noncomplicated hypertensive patients, a significant nocturnal decline in BP occurs, although in hypertensive patients, BP oscillates at a higher level. Nondipper hypertensive patients (ie, those in whom nighttime BP drop is Ͻ10%) have a worse prognosis as a result of increased target organ damage. 2 In this group, significantly greater left ventricular hypertrophy, microalbuminuria, and stroke frequency have been described. 2,3 Determination of ultradian components of BP rhythm (ie, those rhythmic components with a period Ͻ24 hours) has often been used as a tool to improve accuracy of the mathematical analysis of 24-hour rhythm parameters. 4 However, because ultradian rhythms have been shown to vary independently of 24-hour rhythms in various situations, 5,6 the possibility arises that their genesis and regulation could be somewhat independent from those regulating 24-hour rhythmicity.We undertook the present study to compare ultradian rhythm parameters in normotensives and dippers and nondipper hypertensive patients. Because disturbances in 24-hour rhythms were implicated in the physiopathology of hypertension, we also assessed the integrity of 24-hour and ultradian rhythms in the same patient groups. Materials and Methods PatientsAmong 265 patients who underwent ambulatory BP monitoring (ABPM), 180 were included in this study. To be included, each patient's recording should extend for Ն22.5 hours but not Ͼ25.5 hours and with Ͻ5 single measurements missing. Patients were classified as hypertensive if they were under pharmacological treatment or if they showed an average diurnal BP of Ͼ135/ 85...