The length-active tension relation has been previously reported to be decreased or unchanged in hypertensive vessels whereas resting distensibility was unchanged or increased. We found the maximum active stress and the internal ring circumference, in millimeters, at which it occurs (L<sub>max</sub>) to be lower in arterial rings from perinephritic hypertensive dogs than in rings from normotensive dogs. The internal circumferences (length) at which resting force and active force became zero (L₀ and L<sub>min</sub>, respectively) were unchanged. L<sub>max</sub>, L₀ and L<sub>min</sub> were used to normalize length-tension diagrams. Active stress was significantly lower in hypertensive vessels at most of the lengths tested with the diagram normalized to Lmax· When the length-tension diagram was normalized to L<sub>min</sub> there was no difference in the active stress at any of the lengths tested. The length-resting stress curves were identical when the diagram was normalized to L<sub>max</sub> but the curve for hypertensive vessels was higher when the diagram was normalized to L₀. An important characteristic of these length-tension curves is that normalized lengths correspond to the same absolute length in each group of vessels when the reference length has the same absolute value (L₀ and L<sub>min</sub> in this study). This separates differences due to absolute length from differences associated with hypertension. We conclude that perinephritic hypertension in the dog is accompanied by a decrease in resting distensibility of the arterial wall. The results indicate that the choice of reference length may affect the values of stress and tension that are obtained for comparison of length-tension relationships in hypertensive and normotensive blood vessels.