The synthesis of collagen (measured as the formation of non-dialyzable hydroxyproline) and other proteins, and the activity of lysyl oxidase, the enzyme responsible for the formation of cross-links in elastin and collagen, were measured in cultured human aortic smooth muscle cells in the presence of various cortisol concentrations. In addition, the effect of cortisol on the proliferation of the cells was also studied. At concentrations of 10(-6) M and greater, cortisol retarded cell growth markedly, as judged by decreased incorporation of 3H-thymidine by the cells and decreased DNA content of the cultures. At the same concentrations, cortisol increased the synthesis of collagen and other proteins. The magnitude of the increase was similar for collagen and other proteins. The activity of lysyl oxidase was also increased in the presence of 10(-6) and 10(-5) M cortisol, but to a lesser extent than the synthesis of collagen. Other studies using cultured cells have shown that cortisol decreases the synthesis of hyaluronic acid but not of sulphated glycosaminoglycans by smooth muscle cells and does not injure endothelial cells. Considering these findings in the light of the present results, it is suggested that cortisol induces changes in the metabolism of connective tissue macromolecules of smooth muscle cells that can be regarded as atherogenic. However, some other important biological changes associated with atherogenesis (smooth muscle cell proliferation and endothelial injury) do not occur in vascular cells exposed to cortisol.