Documentation of the effective length of action of antihypertensive treatment. The technique of automated ambulatory blood pressure (BP) monitoring offers an innovative means for measuring BP throughout the 24-hour period. Recently available compact monitoring instruments have been shown to be accurate and to provide reproducible measurements of the circadian BP pattern. The monitoring procedure is advantageous in that it minimizes or avoids placebo effects during therapeutic trials. Moreover, its power makes it possible to draw statistically valid conclusions regarding efficacy in fewer patients than would be required if conventional methods were used. This procedure also enhances the diagnosis of hypertension by identifying patients with "office" or "white coat" hypertension, and thereby facilitates assessment of treatment effects in those patients who are truly hypertensive. Automated monitoring measures BP at critical times of the day, including the preawakening and early morning hours, and it enables peak and trough antihypertensive drug effects to be carefully quantified. Since patient compliance appears to be enhanced with once- or twice-daily dosing, antihypertensive agents with long durations of action (24 hours) are of considerable interest. This report reviews some recent studies in which the monitoring technique has been used to measure the efficacy and duration of action of differing antihypertensive drugs. 