For the description of the mechanical properties of the arterial wall, a number of different models and mathematical approaches of varying complexity have been used. Most investigations based on the elastomeric approach have been related to the elastic properties of arteries, while relatively little is known about the arterial wall viscosity (eta w) and its dependence on circumferential wall stress (sigma t) and smooth muscle tone. For this reason we have examined these relationships on segments of the abdominal aorta, the carotid artery, and the tail artery of normotensive rats in vitro under the condition of strong smooth muscle activation induced by norepinephrine (NE), and during smooth muscle relaxation induced by papaverine (PAP). RESULTS 1. For the abdominal aorta, the quotient of the dynamic (Ed) and the quasistatic elastic modulus (ESt) increase from 1.5 to 2.1 under NE, and from 1.2 to 1.5 under PAP, for the carotid artery from 2.8 to 3.9 under NE, and from 1.3 to 1.4 under PAP, when sigma t increases from 10 to 120 kPa. For the tail artery, this quotient is 8.6 under NE and 1.9 under PAP in the low sigma t-range (5 to 60 kPa) and 12.1 under NE and 1.5 under PAP in the sigma t-range between 60 and 120 kPa. 2. For all three types of arteries, Ed and eta w increases with increasing sigma t. At a given sigma t, the values of Ed are virtually independent of frequency, while the values of eta w decrease markedly with increasing frequency. 3. The values of Ed and eta w, obtained under NE and PAP, are virtually identical for a given sigma t-range.