The ventral urogenital sinus (UGS) of control male mice has two rows of 3–4 prostatic buds at birth, but how androgens regulate ventral bud (VB) number and patterning is unclear. VBs in both sexes appeared to be a mixture of prostatic and urethral buds. UGSs from Tfm male and antiandrogen (flutamide)-exposed mice had small VBs, suggesting that initiation of some VBs is androgen independent. Tfm male mice are widely considered completely androgen insensitive yet their UGSs were 5α-dihydrotestosterone (DHT)- responsive. VBs (6–8) were generally distributed bimodally on the left-right axis at both minimal and normal male androgen signaling. Yet control females and DHT-exposed Tfm males had 13–14 VBs, whose left-right distribution was fairly uniform. These results suggest that VB number and distribution respond biphasically as androgen signaling increases from minimal, and that androgens regulate bud specification. Complete VB agenesis by the selective budding inhibitor 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) required high androgen signaling. Developmental Dynamics 239:373–385, 2010. © 2009 Wiley-Liss, Inc.