The urinary excretion of free noradrenaline (NA), adrenaline (A), dopamine (DA), the DA/NA ratio in the urine, plasma renin activity (PRA) and their mutual relationship were investigated in 71 patients suffering from different types of arterial hypertension. In spite of the fact that the mean values of excreted catecholamines, with the exception of pheochromocytoma, lie within the range of values found in healthy controls, certain differences were found in spectrum of excreted catecholamines. In patients with labile, malignant and renovascular hypertension and in pheochromcytoma the higher mean excretion of NA and the low DA/NA ratio was accompanied by the higher PRA in comparison with fixed benign essential hypertension. On the other hand, in hypertension with low PRA (essential hypertension with suppressed renin and Conn's syndrome) a low excretion of NA and high DA/NA ratio was found. There was a significant, if not even very close negative correlation between the PRA and DA/NA ratios both in recumbent and upright position. The rise of PRA on standing up was followed by an increased excretion of NA while the excretion of DA did not change or decreased. Hence the DA/NA ratio when standing up showed a decreasing tendency as compared with values when lying down. Application of the beta-blocker Inderal decreased the PRA and the blood pressure not only in juvenile hypertensive patients with hyperkinetic circulation but also in the early phases of renovascular hypertension. It thus appears that endogenous catecholamines, first of all the ratio between the renin-inhibiting DA and the renin-stimulating NA, participate as one of several factors in the regulation of secretion and of the plasma levels of renin not only in juvenile hypertensive patients with hyperkinetic circulation but also in other types of hypertension.