Uncertainty in defining hypertensive disease makes a prolonged study of blood pressure pattern necessary, using continuous or semicontinuous blood pressure recordings. Its pathophysiological meaning involves data sufficiently indicative of blood pressure profile with reference to the continuous stimulations of different intensity and duration, which are met by subjects both in their daily activities and in the passage from an active life to sleep. Such a parameter is, in fact, an indispensable premise for a correct course of therapy. The aim of our work was at first the detection of a diurnal rhythm in blood pressure, using data obtained in a 24-h ambulatory monitoring away from the conditioning of different activities and daily routine. We have employed three groups of 34 males each. The first group consisted of hypertensive outpatients, while hospitalized subjects comprised the second group. The third group was composed of normal subjects. Furthermore, after finding this system productive, we started studying whether this rhythm of blood pressure could be modified under the influence of a single administered drug dose, and whether information obtained could be easily interpreted. We studied 12 male patients with essential hypertension, which had been untreated. Each patient underwent three 24-h blood pressure ambulatory monitorings. Two different doses of nifedipine (10 and 20 mg) were randomly administered to each of the patients at the beginning of the second and third readings. We observed a significant fall in systolic blood pressure and a minor decrease in diastolic blood pressure after administration of a single 10 mg nifedipine tablet.