To evaluate arterial physiopathology, complete arterial wall mechanical characterization is necessary. This study presents a model for determining the elastic response of elastin (sigma E, where sigma is stress), collagen (sigma C), and smooth muscle (sigma SM) fibers and viscous (sigma eta) and inertial (sigma M) aortic wall behaviors. Our work assumes that the total stress developed by the wall to resist stretching is governed by the elastic modulus of elastin fibers (EE), the elastic modulus of collagen (EC) affected by the fraction of collagen fibers (fC) recruited to support wall stress, and the elastic modulus of the maximally contracted vascular smooth muscle (ESM) affected by an activation function (fA). We constructed the constitutive equation of the aortic wall on the basis of three different hookean materials and two nonlinear functions, fA and fC: sigma = sigma E + sigma C + sigma SM + sigma eta + sigma M = EE. (epsilon - epsilon 0E) + EC.fC.epsilon + ESM.fA.epsilon + eta. [equation: see text] + M.[equation: see text] where epsilon is strain and epsilon 0E is strain at zero stress. Stress-strain relations in the control state and during activation of smooth muscle (phenylephrine, 5 micrograms.kg-1.min-1 IV) were obtained by transient occlusions of the descending aorta and the inferior vena cava in 15 conscious dogs by using descending thoracic aortic pressure (microtransducer) and diameter (sonomicrometry) measurements. The fC was not linear with strain, and at the onset of significant collagen participation in the elastic response (break point of the stress-strain relation), 6.02 +/- 2.6% collagen fibers were recruited at 23% of stretching of the unstressed diameter. The fA exhibited a skewed unimodal curve with a maximum level of activation at 28.3 +/- 7.9% of stretching. The aortic wall dynamic behavior was modified by activation increasing viscous (eta) and inertial (M) moduli from the control to active state (viscous, 3.8 +/- 1.3 x 10(4) to 7.8 +/- 1.1 x 10(4) dyne.s.cm-2, P < .0005; inertial, 61 +/- 42 to 91 +/- 23 dyne.s2.cm-2, P < .05). Finally, the purely elastic stress-strain relation was assessed by subtracting the viscous and inertial behaviors.