The effects of chronic morphine administration on a variety of rat organs were examined. Only the prostates and seminal vesicles were affected by a 3-day period of morphine pellet implantation. In these organs, particularly the seminal vesicles, morphine produced a marked decrease in both the wet and dry tissue weight and a reduction in their secretory activity. This apparent morphine-induced atrophy of these important accessory sex organs was not due to a change in the nutritional status of the animals but rather appeared to be associated with a pronounced (87 percent) reduction in the serum testosterone levels of morphine-treated rats when compared to control levels. The effect of morphine on the secondary sex organs was reversed by withdrawing the animals from morphine administration for a period of 7 days. These data suggest that the role of physiological and endocrinological factors in the narcotic addict's sexual difficulties be examined.