Estrogen sources and targets in the male reproductive tract. Estradiol 17β (E2) is produced in peripheral tissues and delivered via the plasma, but is also synthesized by Leydig cells (LC) in the testicular interstitium. The contribution of E2 from testis to plasma and from the vasculature to the testis is unknown, but it is assumed that most of the lymphatic E2 would be derived from LC. LC and germ cells (GC) contain p450 aromatase in the adult testis. LC may also contribute to the E2 concentrations in the rete testis fluid, but it is more likely that germ cell production of E2 provides the estrogen that will target the efferent ductule epithelium, the region that contains the highest concentration of ER. Less is known of E2 function and targets in the epididymis and vas deferens.