Since heart disease is a common problem in men we want to know about the effects of male hormonal contraception on the cardiovascular system. One way to evaluate these risks is to measure the number of progenitor cells and the types of cholesterol in the blood. Progenitor cells are cells that travel in the blood and go to areas of blood vessel injury to help repair the damage amd may help prevent heart attacks and stokes. Some studies suggest that T administration may increase the number of these cells in the blood but other studies have shown that estrogen may be responsible for this effect. In addition, T and estrogen may affect the amount and type of HDL cholesterol in the blood. This is the "good" cholesterol that is thought to protect people from heart attacks and strokes. Therefore, more studies to test the effects of T and estrogen on progenitor cells in the blood and to understand HDL cholesterol in men receiving testosterone are needed.