Structural changes of blood vessels from hypertensive animals during contraction were studied using light and transmission microscopy. In the arteries, maximally contracted with norepinephrine, vacuoles were found in the media of three categories of mesenteric arteries from two age groups (10-12, 28-30 weeks) of spontaneously hypertensive (SHR) and age-matched normotensive Wistar-Kyoto (WKY) rats. These vacuoles were related to contraction of the smooth muscle cells involving shape change, as they were absent in relaxed vessel walls, and also in muscles that underwent isometric contraction. In the SHR, the number of vacuoles/1,000 microns 2 of media was positively correlated with the degree of contraction of the vessels. Electron microscopic examination of these vacuoles showed that they were the result of herniation (or evagination) of smooth muscle cells into the intercellular space as well as into the neighbouring cells. When the number of herniae per unit cross-sectional area of the contracted media was used as the basis for comparison between SHR and WKY, we found no difference between the two strains of rats at 10-12 weeks of age. At 28-30 weeks, however, the number of herniae was significantly lower in the SHR as compared with age-matched WKY and younger SHR, suggesting that functional changes, that functional changes, probably related to the physical property of the plasma membrane in the smooth muscle cells, have taken place in older SHR.