The cell pathology of experimental hypertensioninduced by endocrine methods was traced by histochemical methods, autoradiography and electron microscopy. Arterial mucopolysaccharide synthesis is increased in rats made hypertensive with desoxycorticosterone and sodium chloride overload, and in rats with adrenal regeneration hypertension. Autoradiographs show a high uptake of radioactive sulphate in the mesenteric arteries but no increased venous uptake. New cell formation in the heart, arteries and kidney was traced with <sup>3</sup>H-thymidine autoradiography in the same experimental models. The most marked increase in <sup>3</sup>H-thymidine labelling occurs in arterial smooth-muscle at sites of plasmatic vasculosis which later evolve into hypertensive necrotising arteritis, and this may be reparative in nature. The more generalised but less marked labelling of smooth muscle is a compensatory response to the high blood pressure. Electron microscopy in adrenal regeneration hypertension indicates that fibrin and platelets leak into the arterial wall at focal segments of plasmatic vasculosis. The electron microscopy of renal biopsies from patients with hypertension due to Cushing’s syndrome reveals a form of vascular hyalinosis different from that described in benign essential hypertension.