Experiments were performed to indentify the series elastic component (SEC) in intact dog carotid artery held at in situ length. The vessels were studied during excitation of the muscle with norepinephrine and after metabolic poisoning with potassium cyanide and sodium iodoacetate. Static circumferential stress-strain curves and stress-quick-release stiffness curves were examined to evaluate Maxwell and Voigt model elements. The vessels were studied at 33, 36, and 39 degrees C. Temperature variations altered active stress, but did not alter connective tissue properties or the Maxwell SEC stiffness. The Voigt model SEC stiffness was altered, but this was secondary to changes in active stress. Thus, most of the SEC is separate from the contractile apparatus. Other vessels were treated with elastase, collagenase, or hyaluronidase to digest the connective tissue components of the wall. Hyaluronidase had no effect on mechanics. Elastase and collagenase altered connective tissue properties, but only elastase unequivocally altered SEC stiffness. This analysis indicated 1) that the carotid artery wall is better represented by a Maxwell model than a Voigt model, and 2) that the SEC in intact carotid artery is primarily elastin.