Haemodynamic ambulatory monitoring was derived from the intra-arterial blood pressure (BP) profile in 10 patients with essential hypertension. Stroke volume (SV) was computed beat by beat according to the following formula: X x PSA x (1 + St/Dt), where X is a correction factor, PSA is the area under the systolic portion of the pressure curve, St is the systolic and Dt the diastolic time. The X value was obtained in each patient by predetermining SV by thermodilution and solving the previous formula by X. The correlation between SV calculation and SV measured independently by thermodilution was highly significant: r values ranged from 0.85 to 0.92 (intercepts close to 0 and slopes close to 1) during different situations (supine, tilt, dynamic and static exercise). In five patients continuous haemodynamic ambulatory monitoring was obtained by applying the formula above to the intra-arterial tracing recorded with the Oxford technique. A computer program was developed in order to get BP, heart rate (HR), SV, cardiac output (CO) and total peripheral resistance (TPR) simultaneously. In these patients, the morning increase of BP was determined by an increase of both CO and TPR. Stroke volume increased slightly during the night, probably as a consequence of a reduced HR.