Vasectomy and vaso-occlusion techniques were used in 47 male rhesus monkeys to maximize and minimize the amount of sperm allowed to escape from the vas into surrounding tissues for up to seventy-two weeks postoperatively. Body weight changes and blood clinical data indicated that the general health of all the monkeys remained good. Normal seasonal changes in body weights and testicular volumes suggested that there were no disturbances to the endocrine system and that the monkeys remained responsive to seasonal environmental stimuli. Vasectomy appears to cause no short-term deleterious effects in the rhesus monkeys, based on observations made during the seventy-two weeks that these monkeys were study after vasectomy. This conclusion agrees with the findings of other investigators.