The purpose of this study was to test the effectiveness of various doses of disodium ethane-1-hydroxy-1,1-diphosphonate (EHDP) in an experimental rabbit model of athero-arteriosclerosis designed by Hass et al. (Amer. J. Pathol., 49 (1966) 739). This model, which involves the feeding of a hypercholesterolemic diet in conjunction with the administration of moderately high doses of vitamin D and nicotine, results in an extensive arterial disease with complicated lesions. EHDP was administered daily by subcutaneous injection at levels of 0.25, 1.0 and 2.5 mg/kg body weight beginning with the initiation of the atherogenic regimen. Results of chemical and histopathological analyses after 8 and 12 weeks of treatment indicate the following: (1) There was a dose-related inhibition of arterial calcification at 8 weeks. At 12 weeks, only the 2.5 mg/kg dose of EHDP resulted in reduced calcification. (2) EHDP administration appeared to influence arterial lipid-containing plaque formation in medium sized arteries at 12 weeks. There was no apparent effect of EHDP administration on serum cholesterol and triglyceride levels. (3) EHDP, at a dose of 2.5 mg/kg/day, inhibited the vitamin D induced hypercalcemia. (4) EHDP administration at 2.5 mg/kg/day almost totally inhibited the thromboarteritis accompanying this disease. (5) The data thus indicate that if arterial calcification is inhibited, the other morphological effects of this treatment regime are also inhibited. This effect occurred even though serum lipid levels were unaffected. The data therefore emphasize the role of calcification in the pathogenesis of this type of experimental atherosclerosis and perhaps in human disease as well.