The circulating level of free androgens (testosterone and dihydrotestosterone) in fetal rabbit was reduced by active immunization of mothers against testosterone. The level of testosterone and dihydrotestosterone binding to circulating proteins was about 100% in immunized fetuses and newborns. The testosterone and dihydrotestosterone binding capacities were higher than their circulating concentrations. Macroscopically, all newborns exhibited feminine external genitalia. Histological examinations showed the presence of complete wolffian ducts differentiated in epididymis, deferent ducts and seminal vesicles with normal structure. Prostatic development was strongly inhibited; anterior prostates were present but lateral and posterior prostates were frequently absent. In most animals the masculinization of the genital tubercle was prevented: 60% of males exhibited feminine external genitalia, 40% severe hypospadias. These results suggest: 1 degrees that fetal free circulating androgens (testosterone and/or dihydrotestosterone) induced masculinization of urogenital sinus and genital tubercle; 2 degrees that testicular testosterone was responsible for stabilization and differentiation of the wolffian duct.