In developing mammalian males, conversion of the Wolffian ducts into the epididymides and vasa deferentia depends on androgen secretion by the testes, whereas in females these ducts remain in a vestigial form or regress. However, there is continuing uncertainty whether the androgen needs to be delivered locally, either by diffusion from the adjacent testis or, by secretion into the lumen of the duct, or whether circulating androgens maintain and virilize the Wolffian ducts. To resolve this uncertainty, we transplanted either day 0-2 or day 8-9 post-partum testes beneath the flank skin of three groups of neonatal (days 0-1) female tammar wallabies, where they developed and secreted physiological levels of hormones. The Wolffian ducts of all these females were retained and had formed extensive epididymides when examined at days 25, 34 and 87 after birth. In the two older groups of females, sampled after the time of prostatic bud formation, the urogenital sinus was virilized and there was extensive prostatic development similar to that of normal males of the same age, showing that androgen secretion had occurred. Virilization of the Wolffian ducts occurred during an early but short-lived window of sensitivity. This study provides the first clear evidence that under physiological conditions virilization can be mediated by circulating androgen.