Renal tissue obtained by means of biopsy or nephrectomy from 11 patients with various degrees of arterial hypertension was studied by electron microscopy. A distinct correlation was found between the level of arterial pressure and the degree of arteriolar lesions. According to the author, arteriolar damage in hypertension develops in two stages: 1. Hypertrophy of initmal and medial cells and hyperplasia of intracellular organelles. 2. Degeneration of the arteriolar wall with deposition of an electron dense material (hyaline) under the basal membrane and in the intercellular matrix of the media. The first stage may be regarded as an enhancement of the compensatory and adaptive processes in the arteriolar wall. The second stage is a manifestation of irreversible changes associated with the progression of the disease. The sequence is apparently common to all parts of the arteriolar system and to a certain extent it may be considered typical for the development of hyaline arteriolosclerosis. The penetration of blood plasma components and their deposition in the arteriolar wall depends, according to the author, on an increased intercellular and cellular permeability, and hypertension evidently enhances this process.