The ability of the female urogenital sinus to respond to androgens in forming prostate was determined by growing 13- to 18-day old embryonic female urogenital sinuses and vaginas from 1- to 30-day old mice as grafts to male hosts. All embryonic unrogenital sinuses as well as vaginas from 1-day old mice were responsive to androgens and formed prostate, whereas vaginas from mice 5- or more days old never formed prostate. To determine which tissue, the epithelium or stroma, accounts for the age-dependent loss in responsiveness of the vagina to androgens in forming prostate, recombinations composed of epithelium and stroma from 16-day old embryonic urogenital sinuses and vaginas from 1- to 20-day old mice were grown as grafts to male hosts. The developmental response of these recombinants demonstrated that the age-dependent loss in responsiveness of the intact vagina to androgens results from an age-dependent loss in the ability of vaginal stroma to participate in prostatic morphogenesis. These data emphasize the importance of stromal factors during prostatic morphogenesis and the relationship of temporal factors to developmental properties of urogenital stroma.