Circadian blood pressure variations were studied in 110 normotensive (blood pressure less than or equal to 140/90 mmHg) and 142 untreated essential hypertensive (blood pressure greater than or equal to 160/95 mmHg) subjects. Measurements were carried out under full ambulatory conditions by the oscillometric method, using an automatic device (SpaceLabs 5200). Readings taken between 9.00 a.m. and 7.00 p.m. were defined as daytime values (activity), and those taken between 11.00 p.m. and 7.00 a.m. as night-time values (sleep). Blood pressure was significantly higher during the day than at night. The day-night differences were significantly higher in the hypertensive than in the normotensive subjects. There was a significant correlation for both systolic blood pressure (SBP) and diastolic blood pressure (DBP) between the day-night differences and DBP values at rest. The difference between the normotensives and hypertensives was smaller when the day-night difference was calculated as a percentage of the daytime blood pressure, but the SBP difference remained significantly higher in the hypertensives. The number of subjects without a nocturnal blood pressure decrease or with a decrease of less than 5-10 mmHg was not statistically different between the normotensives and the hypertensives. These results are a contribution towards an understanding of day-night blood pressure differences in hypertensive and in normotensive subjects. A knowledge of the 24-h blood pressure profile in the individual patient is important because abnormal profiles may be linked with various etiologies responsible for dysregulation, and might have prognostic implications.