The data on plasma renin activity (PRA) obtained by catheterization of renal veins in 272 patients with essential arterial hypertension (EH) are reported. In order to exclude a secondary nature of hypertension, all patients were hospitalized and submitted to the extensive clinical and laboratory examinations (according to the protocol) including renal angiography. The aim of the work was to get evidence whether in the apparently homogeneous group of patients with EH there are differences in renin activity between the renal veins giving the quotient (Q) higher than 1.5, and possibly to recognize the clinical meaning of the differences of Q found. In the examined patients as a whole regardless of PRA, 50 of them (18.3%) showed Q greater than or equal to 1.5 and among them in 16 (5.8%) patients Q greater than or equal to 2 was found. The patients were divided according to PRA into those with low, normal and high PRA. In the hyperreninemic group of patients no Q greater than or equal to 2.0 has been found. In the hyporeninemic group 9 patients (9.9%) exhibited Q greater than or equal to 2.0. Although, one does not expect in EH to find differences in PRA between left and right renal vein, we have found the values of Q greater than or equal to 1.5 in nearly 1/5 of our patients. These differences found could be resulting from the existence of asymmetrical angiosclerotic or other renal pathological changes that can not be examined by the available clinical methods.(ABSTRACT TRUNCATED AT 250 WORDS)