The circadian rhythm of arterial pressure (AP) is not a passive consequence of the impact of exogenous factors. Endogenous mechanisms play an important role in the generation and maintenance of AP rhythm. The adaptation of the exogenous components of AP rhythm to the demands of the environment is modulated by the circadian-time-dependent responsiveness of the biologic oscillator. A neuronal network in the rostral hypothalamus including the suprachiasmatic nucleus is implicated in the generation of AP rhythm, in the modification of the rhythm amplitude (possibly due to homeostatic constraints), and in the regulation of its phase. The central sympathoexcitatory pathway to the upper thoracic cord plays a crucial role in the maintenance of normal circadian AP rhythm. The circadian pattern of AP is influenced also by hormonal factors such as the hypothalamic-pituitary-adrenal and the hypothalamic-pituitary-thyroid axes, the renin-angiotensin-aldosterone system, opioids, and various vasoactive peptides. The circadian variations of AP depend on physiological state--sleep and wakefulness, pregnancy, work, and senescence (primary aging). In some essential hypertensive patients and in patients with secondary hypertension the nocturnal fall in AP is reduced or absent (nondippers). Target-organ damage is more advanced in nondippers than in dippers. The occurrence of cardiovascular events exhibits a prominent circadian pattern, with events more frequent in the morning (06:00-12:00 h).