For the first 28 days of the study, the men ate a diet high in saturated fat. Then, half of the men switched to an NCEP-1 diet and the other half switched to a Mediterranean diet for the next 28 days. After this, the men who had been on the NCEP-1 diet switched to the Mediterranean diet and vice versa for the last 28 days of the study. At the end of each of the diets, the researchers collected blood samples to measure cholesterol and other fats in the blood. They also studied the endothelial function of each man using special ultrasound tests of the blood vessels.