Rats were injected with deoxycorticosterone (DOC) and salt was added to the drinking water (DOC/NaCl) for 3 weeks. Approximately half of the animals became hypertensive (DOC-H), whereas the remainder showed no increase in blood pressure (DOC-N) compared to age-matched, untreated controls. Morphometric analysis of the alterations in the arteries of the mesenteric bed was carried out in vessels fixed at maximum relaxation. Alterations were observed in the arteries, some of which were related to hypertension, but not to treatment, and some were due to treatment alone. The alterations within the arteries of the mesenteric bed depended in part on the type of artery, i.e. elastic, muscular or arteriolar. An increase in lumen area, in intimal area, and in the area of the media was seen in all types of arteries from DOC-H, but not in either group of normotensive animals. The medial hypertrophy was positively correlated with the increase in blood pressure; and was due to an increase in the number of smooth muscle cell layers in the elastic and muscular arteries, and probably to smooth muscle cell hypertrophy in the arterioles. The adventitial area was increased only in the elastic and muscular arteries. Endothelial injury, degeneration of the basement membrane, loss of heparan sulfate proteoglycan and intimal edema was observed in all the DOC/NaCl-treated animals, but was more severe in those which were also hypertensive. Hypertrophy of the heart and kidneys were observed in both normotensive and hypertensive DOC/NaCl-treated animals. These data indicate that changes in the rat cardiovascular system can be induced by DOC/NaCl treatment in the absence of hypertension, but also that hypertension is associated with specific arterial structural alterations, which vary according to the type of artery.