Abstract: There is variation in the sexual performance of male mammals. The purpose of this invention is to help identify high sexual performing males, particularly for livestock such as sheep. Testosterone is necessary for the normal execution of male behavior, but baseline levels are not predictive of libido. When a male mammal is given an injection of naloxone (an endorphin antagonist), his testosterone or LH response to the injection predicts whether he is a sexually active or inactive individual. This test is based on the premise that libido is more closely linked to the ability to secrete testosterone rather than baseline concentrations. Naloxone challenge may be used as a general method to predict libido in male mammals.