1. Propranolol and metoprolol were given in the diet of spontaneously hypertensive (SH) rats during the development of hypertension.
2. At the early established phase of hypertension, protein synthesis and protein composition of the left ventricle, aorta, mesenteric artery and vena cava were measured.
3. The left ventricle, aorta and mesenteric artery from the SH rats were hypertrophied compared with control Wistar—Kyoto rats, and protein composition was not dramatically changed. The increased protein content could only in a few cases be assigned to an increased protein synthesis at this age.
4. Both β-adrenergic antagonists prevented the hypertrophy of aorta and mesenteric artery but not in heart.