1. Sequential changes in the ionic composition of the aorta and skeletal muscle were followed during 1, 2 and 4–6 weeks in 30 rats given deoxycorticosterone (DOC) and salt supplemented with potassium chloride. Twenty-one rats, drinking water, were used as controls.
2. Twenty-five per cent of the test rats were hypertensive after 1 week, 60% after 2 weeks and 100% after 4–6 weeks.
3. Muscle potassium fell in all test rats by an average of 15%. In contrast, aortic potassium fell by 19% only in those rats which did not develop hypertension after 1 week.
4. Total and non-inulin sodium and water of the aorta were normal in rats which remained normotensive after 1 or 2 weeks and high in those which became hypertensive during the same period.
5. Total sodium and water content of the aorta were also high in rats which were hypertensive at 4–6 weeks. However, because of simultaneous expansion of the inulin space, non-inulin fractions were normal in this group.
6. Results suggest that vascular ionic changes participate in the pathogenesis of DOC-salt hypertension through more than a single mechanism.