Segments of carotid and tall artery, and thoracic aorta from control and hypertensive animals (DOCA + salt) were used for the study of mechanics and/or chemical composition. Pressurehyphendiameter measurements were made on intact segments under conditions of actire (145 mMhyphenK+) and passive (Ohyphenca++and 2 mMhyphenEGTA) smooth muscle. Segments were used for chemical analyses of connective tissue content, water spaces, and electrolyte content. The passive stiffness of carotid and tail arteries increased monotonically with time. The carotids showed significant changes after two weeks of hypertension while the tail arteries only after 12 weeks. The collagen and total connective tissue content of the hypertensive arteries was decreased while collagen/elastln was unchanged. Smooth muscle activation produced larger changes in diameter of hypertensive arteries especially at higher values of transmural pressure. Maximum active force development was increased in carotid arteries at each time period from +2 weeks on while it was Increased for the tall arteries only at +2 and +4 weeks. Relative cellular volume of these arteries was monotonically increased with hypertension. Maximum active force normalized to cellular content was not significantly different for carotid arteries from control and DOCA rats. For hypertensive tall arteries normalized on this basis force development remained elevated at +4 weeks but was significantly reduced at +12 weeks. Not all of the responses to smooth muscle activation are monotonic with duration of hypertension, nor can all of these changes be explained on the basis of changes in cellular volume. (Hypertension 4: 27–38, 1982)