The circadian rhythm of blood pressure derived from continuous recordings of blood pressure in hypertensive subjects, provides an accurate and repeatable method of describing blood pressure on a temporal scale which can be used to evaluate the effects of anti-hypertensive drugs. It has been postulated that the rapid rise of pressure between 0600 and 1000 hr is due to intrinsic sympathetic activity mediated at the peripheral arteriole by alpha-adrenoreceptors. Drugs which block beta-adrenoreceptors do not appear to influence blood pressure at this time, no matter how frequently they are administered or how long-acting they may be. Meteroprolol and sotolol are beta-adrenoreceptor blocking drugs which appear to be different in that they produce a considerable fall in nocturnal blood pressure, which is similar to the effects of alpha-adrenoreceptor blocking agents. The physiological and clinical significance of these findings remains to be defined, but they emphasise the need for full and complete information on blood pressure changes before the most effective method of blood pressure reduction can be defined.