To investigate the biochemical mechanisms involved in the cataract induced by lovastatin, a commonly used cholesterol-lowering agent. The effects of lovastatin on lens transparency and on lens epithelial cell proliferation and structure have been investigated using organ-cultured rat lenses and cultured epithelial cells from human and rabbit lenses, respectively. Lens histologic and morphologic changes were recorded microscopically. Small GTP-binding protein profiles were determined by [alpha-32P] GTP overlay assays. Rat lenses organ cultured for 7 days with lovastatin, a 3-hydroxy-3-methylglutaryl CoA reductase inhibitor, developed frank subcapsular opacity. Lens epithelial cells (both human and rabbit) demonstrated extensive morphologic changes and inhibition of proliferation when treated with lovastatin. Histologic sections of lovastatin-treated lenses showed partial to complete degeneration of the central epithelium, distortion of elongating epithelial cells, and extensive vacuole formation in the equatorial regions of the cortex. Supplementation of the medium with DL-mevalonic acid (a precursor of isoprenoids whose synthesis is inhibited by lovastatin) prevented the lovastatin-induced changes in whole lenses or in lens epithelial cell cultures, whereas supplementation with cholesterol had no such effect. GTP-binding proteins accumulated in the soluble fractions of lovastatin-treated lens epithelial cells. This was consistent with a blockade in isoprenylation preventing normal association with membranes. The findings suggest that impairment of the function of small GTP-binding proteins, due to a lovastatin-induced blockade in their isoprenylation, affects lens cell structure and proliferation in tissue culture and induces lens opacity in organ culture. These findings are consistent with the proposed roles of small GTP-binding proteins as molecular switches that regulate fundamental cellular processes, including growth, differentiation, and maintenance of cell structure.