In briefExhaustive endurance training may suppress testicular function. This tentative conclusion was drawn from a study of six endurance athletes who underwent 2 weeks of prolonged running and cycling. No significant changes were noted for strength, weight, or body fat. However, persistent fatigue developed in all subjects. Their plasma testosterone decreased 17% (from 673 to 558 ng/dl, p<.05), and sperm count decreased, though not significantly. All reported a decline in sexual activity and sex drive, which may be related to the development of general fatigue and could be an early sign of overtraining stress.