The development of aortic and coronary atherosclerotic plaques were investigated in roosters fed an atherogenic diet with and without the addition of Valium (0.2 mg/kg twice daily) over a period of 5 months, as a step toward understanding the role of emotional factors in atherogenesis. Plasma levels of cholesterol and triglycerides, hemodynamic parameters, and body weight were measured. There was a progressive, and quantitatively similar, body weight gain in all birds. In addition, there were no significant differences in values for blood pressure, cardiac output, and heart rate between the various experimental groups. Conversely, birds receiving the atherogenic diet, as well as those receiving the atherogenic diet along with Valium, exhibited a marked hypercholesterolemia which reached a peak of 600 mg/dl in 4-6 weeks, before decreasing to between 200 and 300 mg/dl by the 10th week. Plasma triglyceride levels followed a qualitatively similar pattern as plasma cholesterol. Those birds fed an atherogenic diet alone developed atherosclerotic lesions on the aortic surface which was more pronounced on the abdominal than the thoracic aorta. Aortas from birds given Valium along with the atherogenic diet were completely free of lesions despite the fact that plasma cholesterol and triglyceride levels were not significantly different in the two groups. Histological sections of coronary arteries showed severe lesions in 4 out of 7 birds which were fed the atherogenic diet alone, whereas the birds given Valium along with the atherogenic diet had only an occasional slight lipid deposit. It was concluded that Valium provides some protection against the development of atherosclerosis in roosters fed an atherogenic diet.