Abstract.Forty patients with uncomplicated essential hypertension were investigated with respect to diurnal variability of arterial pressure (indirect recordings), intra‐arterial pressure, cardiac output, plasma volume, renal plasma flow and glomerular filtration rate. Extracellular volume was estimated in 17, plasma renin concentration in 33 and vector‐cardiograms were recorded in 27 patients. Treatment was discontinued at least a fortnight before and sodium intake was standardized.—Blood pressure varied across a wide range. Variability (lability) of blood pressure was quantified by expressing the difference between highest and lowest automatic blood pressure readings as a percentage of the highest reading. Cardiac output correlated with variability of blood pressure, blood volume and renal blood flow.—Plasma renin concentration was correlated with renal vascular resistance and filtration fraction.—QRSmagnitude appeared to be related with the level of arterial pressure.—Haemodynamic variables exhibited a definite relationship with age, deviating in part from distribution according to age in normal