Changes in the smooth muscle cell compartment (SMCC) of the media layer of the aorta were studied in spontaneously hypertensive (SHR) and in normotensive rats (WKY) of both sexes between 3 to 18 weeks of age. Up to 7 weeks of age, development of the SMCC occurred in males and females of the two strains both through a massive increase in cell number and in cell size. In SHR after 7 weeks of age, the development of the SMCC was due to a marked increase in cell size together with an increase in cell number. In contrast during the same period, the development of the SMCC in WKY was associated almost exclusively with an increase in cell size. It is concluded that the presence of a greater number of larger smooth muscle cells contributes to the hypertrophy of the arterial wall of hypertensive animals.