Moving rapidly from a supine to a standing posture is a common daily activity, yet a significant physiological challenge. Syncope can result from the development of initial orthostatic hypotension (IOH) involving a transient fall in systolicdiastolic blood pressure (BP) of >40/20mm Hg within the first 15 s, andor a delayed orthostatic hypotension (DOH) involving a fall in systolicdiastolic BP of >20/10mm Hg within 15min of posture change. Although epidemiological data indicate a heightened syncope risk in the morning, little is known about the diurnal variation in the IOH and DOH mechanisms associated with postural change. The authors hypothesized that the onset of IOH and DOH occurs sooner, and the associated cardiorespiratory and cerebrovascular changes are more pronounced, in the early morning. At 06:00 and 16:00h, 17 normotensive volunteers, aged 26±1 yrs (mean±SE), completed a protocol involving supine rest, an upright stand, and a 60° head-up tilt (HUT) during which continuous beat-to-beat measurements of middle cerebral artery velocity (MCAv), mean arterial BP (MAP), heart rate, and end-tidal PCO2 (PETCO2) were obtained. Mean MCAv was ∼12% lower at baseline in the morning (p≤.01) and during the HUT (p.30). In conclusion, although there is a marked reduction in MCAv in the morning, there is an absence of diurnal variation in the onset of and associated physiological responses associated with IOH and DOH. These responses, at least in this population, are unlikely contributors to the diurnal variation in orthostatic tolerance.