The relationship between the arterial blood pressure and the composition of the tunica media in spontaneously hypertensive, Wistar-America and Wistar-Kyoto rats was examined. The systolic blood pressure of the spontaneously hypertensive rat, measured through an intra-arterial cannula, was significantly higher than that of the controls from 5 weeks and became hypertensive by 18 weeks of age. However, the content of the tunica media of the abdominal aortal and renal artery of the spontaneously hypertensive rats, as revealed by EM stereological analysis, and measurements of the thickness of the wall and diameter of the lumen, were not significantly different from the controls, at least up to the age of 18 weeks. The result of the present investigation suggests that the elevation of arterial blood pressure in the spontaneously hypertensive rat during the first 18 weeks of postnatal development is not a result of stereologically detectable changes in the composition of the tunica media of the abdominal aorta and renal artery.