Oxandrolone is a 5 alpha-reduced anabolic steroid that is administered for the treatment of short stature disease in children. It is a commonly used substance beginning as early as prepuberty by some individuals who are seeking to enhance athletic performance or personal appearance. Because of the lack of data on the effects of anabolic steroids on the reproductive system, we have examined the effects of oxandrolone treatment on reproductive development in male rats with treatment beginning two days after weaning. Male, Sprague-Dawley rats (N = 12) received a daily subcutaneous injection of oxandrolone (32.7 mumol.kg-1.day-1) and the control group (N = 12) received vehicle only (dimethyl sulfoxide). Treatment began at age 23 days and continued to 60 days of age. The weights of the testes, prostate glands, and seminal vesicles in the treatment group were 69%, 50% and 29% below control levels, respectively and were all significantly decreased (p less than 0.01). Testicular testosterone production in a 3-h incubation was inhibited in the treated animals to 1.3% of control values (p less than 0.001). Serum FSH (11.7% of control) and LH (undetectable) in the treated animals were both significantly less than controls. Histological findings indicated an arrest of advanced spermatids and a severe depletion of Leydig cells in the interstitial compartment. It was concluded that treatment of immature male rats with oxandrolone results in effects on the adult male reproductive system which are profound and occur at several levels. The most likely affected sites are the hypothalamus, pituitary gland, and the Leydig cells.