1. Arterial and glomerular lesions were studied in seventy-eight specimens from patients with severe or malignant primary or secondary hypertension by transmission and scanning electron microscopy and light-microscopy and immunofluorescent microscopy.
2. The lesions were classified as compensatory, decompensated and reparative. The arterial lesions of primary and secondary hypertension were qualitatively similar. Each of the microscopy techniques contributed to a better understanding of the changes.