The effects of chronic administration of estradiol and testosterone on the passive mechanical properties of carotid arteries were determined using castrated and noncastrated male rats. Blood vessel segments were removed from the animals and mounted in a bath at in vivo length. Measurements of external diameter and transmural pressure were made in response to inflation in the presence of smooth muscle inhibitors. These data were used to determine stress-strain relations and values of incremental elastic moduli for the various arteries. Relative to arteries from control animals, passive tangential stress-strain curves for arteries from estradiol treated rats were found to be shifted significantly to the right, i.e., to larger strains. Similar data for testosterone-treated arteries were shifted to the left, but the effect was smaller than the former one and only significant for values of tangential stress greater than 2 × 10<sup>–6</sup> dyn/cm<sup>2</sup>. At specific values of wall strain, estradiol produced a decrease in incremental elastic modulus relative to the arteries from control animals, while testosterone produced an increase in elastic modulus which was only significant for strains > 0.7. Quantitatively, similar results were found in castrated and noncastrated rats. These results are consistent with the concepts that estradiol decreases the stiffness of arteries while testosterone to a smaller degree increases the latter through the effects of these steroids on connective tissue accumulation in the arterial wall.