Causes of hypertension have been well scrutinized, whereas the secondary, disabling effects of high blood pressure are less well investigated. We have used a rat model of hypertension and developed a technique to study the secondary vascular smooth muscle component of the disorder. Banding patterns of myosin heavy chain isoforms from rat aortae were examined using denaturing electrophoresis, Western blotting, immunochemical identification, and degradation studies. Myofibrillar ATPase activities were also measured. Left ventricular hypertrophy and hypertension were induced in rats by aortic banding just proximal to the renal artery. Aortic banding increased the heart weight/body weight (mg/g) ratio from 2.8 to 3.8 and mean aortic weight by 53%. Two distinct myosin heavy chain isoforms, molecular masses of 204 and 200 kDa, were identified by 4% sodium dodecyl sulphate-polyacrylamide electrophoresis of crude aortic extracts from normal rats in a relative molar ratio of 1.54:1. The development of significant thickening of the aorta was marked by substantial increases in aortic wall smooth muscle content but was not associated with any changes in distribution of the isoforms. The band patterns obtained on gel electrophoresis were not the result of contamination by other proteins, as Western blotting studies with specific antibodies demonstrated that the isoforms were smooth muscle in origin and were not derived from nonmuscle myosin sources. Myofibrillar ATPase activity of aortic smooth muscle from hypertensive rats was increased. It is suggested that this increase may be the result of post-transcriptional alterations of one or more sarcomeric proteins involved in the regulation of smooth muscle contraction.