Vince Gironda, a well-known body-building trainer in Studio City, California, once admonished bodybuilders to avoid running because it led to overtraining and "loss of male hormone". At the time Vince gave his warning (in the late 1960s), the idea seemed farfetched; but later research confirmed some of Vince's theory.
Short-term, maximal exercise, such as weight training, appears to raise testosterone levels. Extended endurance training - such as marathon running - definately lowers testosterone in men.
(Muscle & Fitness, June 1990)