Measurements of trough blood pressure in a clinic setting have been the traditional method of assessing the efficacy of antihypertensive agents. The duration of action of antihypertensive drugs has been assessed by calculation of a trough-to-peak ratio; drugs with a trough-to-peak ratio greater than 50% are typically given once-a-day indications. However, the use of clinical measurements to assess antihypertensive agents can be misleading. Ambulatory blood pressure monitoring is a simple technique that provides accurate and reproducible data on both the efficacy and duration of action of antihypertensive agents. Although several complicated techniques have been used for the analysis of ambulatory blood pressure data, studies have demonstrated that calculation of simple blood pressure means (24 h mean, day-time mean and night-time mean) will provide all the data required to assess the efficacy of a drug. Calculations of systolic and diastolic load also provide useful information, and the index correlates closely with target-organ damage. Assessing the reduction of blood pressure during the last 2-6 h of the dosing interval provides critical information on the duration of action of agents with once-a-day dosing. Trough-to-peak ratio can also be calculated from an ambulatory blood pressure monitor. Furthermore, a simple line graph constructed from hourly means makes available, at a simple glance, a large amount of information about a drug. The reproducibility of ambulatory monitoring, together with the absence of placebo effect and the ability to exclude patients with white-coat hypertension, make the technique an extremely powerful tool for the assessment of antihypertensive agents that clearly provides more data on the efficacy and duration of action of an antihypertensive agent than do traditional clinical measurements.