In this study the results are presented of the haemodynamic and endocrinological inventarisation of 207 patients with essential hypertension. After establishing the relations of the various parameters with age (part IIA), interrelations between these variables are described (part IIB). The general finding is an increase in total peripheral vascular resistance early in the course of essential hypertension. In particular, this rise in resistance is found in the renal vasculature. In part III emphasis is given to the natural history of essential hypertension on the basis of a follow-up study. Again a rise in renal vascular resistance is found. As pointed out in the discussion (part IV) the cause of the abnormal resistance remains unknown. Emphasis is given to the renin-angiotensin system, which plays only a minor role. Here the factors regulating long-term renin secretion are discussed in favour of the baroreceptor theory.