Spontaneously hypertensive (SHR), Wistar-Kyoto (WKY), and Sprague-Dawley (SD) rats at the ages of 4, 8, 12, 16 and 24 weeks were used to examine the effects of age on the density of endothelin-1 (ET-1) binding sites in aortic smooth muscle cells and systolic blood pressure (SBP). The SBP of the 3 different rat strains was measured, and the maximum binding value (Bmax) and dissociation constant (Kd) of ET-1 binding sites in smooth muscle cells of the thoracic aorta were determined. The results showed that the SBP and Bmax values of SHR, WKY and SD rats increased with age; the SBP and Bmax value at each corresponding age were significantly higher in SHR than in WKY and SD rats, however, there was no significant difference between WKY and SD rats. The relationship of age vs SBP, age vs Bmax, and Bmax vs SBP showed significantly positive correlation in all 3 rat strains. The regression line in the Bmax of endothelin binding sites against SBP in the 3 different rat strains presented a similar slope. These results indicate that SBP, which increased with age, could be related to an increased density of ET-1 binding sites on vascular smooth muscle cells in these 3 different rat strains.