Although a large number of antihypertensive drugs have been approved for once- or twice-daily dosing, no standardized set of evidence is required to demonstrate that the blood pressure reduction is sustained for 24 h. The recent literature on calcium antagonists was therefore searched for the arguments usually put forward to objectify a long duration of action. Most studies relied on ambulatory blood pressure monitoring. However, several published reports were difficult to interpret for a variety of reasons, such as: (1) a non-blinded non-randomized study design; (2) a statistical analysis, which was discordant with the study design; (3) analyses confined to the 24 h, day-time and night-time pressure means; (4) the absence of a baseline adjustment and/or formal statistical testing; (5) the "post hoc" subdivision of patients into responders and non-responders; and (6) the absence of a well-specified time-frame linking drug intake to the observed antihypertensive effects. According to the authors' interpretation, amlodipine, diltiazem SR (modified release), felodipine SR, isradipine SR, nifedipine SR, nitrendipine and verapamil SR have all been confirmed to reduce both the conventional and the 24 h blood pressure. Some studies went beyond the 24 h blood pressure means and also presented separate results for the day-time (or awake) and night-time (or sleeping) periods, or investigated the blood pressure reduction at the end of the dosing interval, or compared the diurnal blood pressure profiles on different treatments.(ABSTRACT TRUNCATED AT 250 WORDS)