Testicular androgens induce formation of the male urogenital tract in all mammals. In marsupials male development occurs after birth and over a prolonged period. For example, in the tammar wallaby virilization of the Wolffian ducts begins by day 20, prostate formation begins about day 25, and phallic development starts after day 80 of pouch life. Between days 20 and 40 5α-androstane-3α,17β-diol (5α-adiol) is formed in tammar testes and secreted into plasma. Administration of 5α-adiol to pouch young females induces urogenital sinus virilization by day 40 and formation of a mature male prostate and phallus by day 150. 5α-Adiol is synthesized in pouch young testes by two pathways, one involving testosterone and dihydrotestosterone and the other 5α-pregnane-3α,17α-diol-20-one and androsterone as intermediates, both utilizing steroid 5α-reductase. In target tissues 5α-adiol acts via the androgen receptor after conversion to dihydrotestosterone but may have other actions as well. Whether 5α-adiol plays a role in male development in placental mammals is uncertain.