Sympathetic stimulation is accompanied by a reduction of arterial distensibility, but whether and to what extent elastic and muscle-type arterial mechanics is under tonic sympathetic restraint is not known. We addressed this issue by measuring, in the anesthetized rat, the diameters of the common carotid and femoral arteries with an echo-Doppler device (NIUS 01). Blood pressure was measured by a catheter inserted contralaterally and symmetrically to the vessel where the diameter was measured. Arterial distensibility over the systolic-diastolic pressure range was calculated according to the Langewouters formula. Data were collected in 10 intact (vehicle pretreatment) and 9 sympathectomized (6-hydroxydopamine pretreatment) 3-month-old Wistar-Kyoto rats. Compared with the intact animals, sympathectomized rats showed a marked increase in arterial distensibility over the entire systolic-diastolic pressure range. When quantified by the area under the distensibility-pressure curve, the increase was 59% and 62% for the common carotid and femoral arteries, respectively (P<.01 for both). In the femoral but not in the common carotid artery, sympathectomy was accompanied also by an increase in arterial diameter (+18%, P<.05 versus intact). Therefore, in the anesthetized normotensive rat, sympathetic activity exerts a tonic restraint on large-artery distensibility. This restraint is pronounced in elastic vessels and even more pronounced in muscle-type vessels.