Intraarterial blood pressure monitoring has shown the circadian rhythm of blood pressure control. Blood pressures tend to be highest in the morning before falling gradually during the day to a nadir at 3:00 a.m. There is a slight rise in the late afternoon that may correspond to patients' attendance at hospital for calibration of the equipment. There is a small rise in the blood pressure before awakening, and after arousal there is a rise in blood pressure to the peak level of the morning. In this article, we examine the effect of a variety of antihypertensive agents on this rhythm. In general, β-adrenoceptor blockers appear to have less effect on nocturnal blood pressure and surge in pressure after arousal, while vasodilators, particularly α-adrenoceptor blockers, have a pronounced effect. These findings indicate that the rise in blood pressure before awakening and the rapid rise upon arousal appear to be due to increased α-adrenoceptor activity.