Surgical constriction of the subdiaphragmatic aorta in young adult male rats induced a 54 per cent hypertrophy (39 per cent thickening) of the wall of the thoracic aorta in 8 days, with no increase in the number of elastic lamellae. Morphometric analysis of the aorta wall showed volume increases of its component smooth muscle cells (57 per cent) elastic laminae (30 per cent), collagen (136 per cent), and endothelium plus subendothelial space (40 per cent). Mean cell volume of smooth muscle cells increased 55 per cent from 272 +/- 11 to 421 +/- 21 cu. mum. (p less than 0.005). The number of smooth muscle cells per centimeter of length of thoracic aorta was 4.2 +/- 0.3 x 10(6) in both control and experimental animals. Thus, aortic medial hypertrophy following aortic stenosis included enlargement but no proliferation of smooth muscle cells. Absolute volume increases in the average smooth muscle cell included the nucleus (27 per cent), myofilaments (47 per cent), mitochondria (62 per cent), rough endoplasmic reticulum (246 per cent), smooth endoplasmic reticulum (23 per cent), and caveolae (37 per cent).