A prospective study of effects of vasectomy on the plasma levels of testosterone, estradiol, follicle-stimulating hormone (FSH), and luteinizing hormone (LH) was instituted. Postvasectomy plasma hormone levels were compared with prevasectomy, base line levels. Beginning at 6 months after vasectomy, mean plasma testosterone levels demonstrated a statistically significant elevation, mean plasma estradiol levels were lower, mean plasma LH levels were elevated, and mean plasma FSH levels were unchanged. By 2 years after vasectomy, a slight plasma FSH elevation had occurred, plasma estradiol levels had returned to base line, and plasma testosterone and LH levels remained elevated. These changes, although significant statistically, did not exceed the normal ranges found in normal adult males in our laboratory and were of unclear physiologic significance. Thus, it can be concluded from this study of 56 men studied for 2 years, 148 men studied for 1 year, and 182 men studied for 6 months after vasectomy that no adverse hormonal effects of vasectomy have been demonstrated.