It has been shown previously that wild-type urogenital sinus mesenchyme can induce the formation of prostate-like glandular structures in urinary bladder epithelium derived from adult Tfm (testicular feminization) mice. Total DNA synthesis within these tissue recombinants has been shown biochemically to be androgen sensitive. To determine which tissue (epithelium or stroma) accounts for this androgen-dependent DNA synthesis, an autoradiographic study was performed with tissue recombinants composed of rat wild-type urogenital sinus mesenchyme (UGM) associated with bladder epithelium from either wild-type (BLE) or Tfm mice (Tfm BLE). Both types of recombinants were grown under the kidney capsule of male athymic nude mice for 4 weeks. The hosts were then castrated, and 2 weeks later were treated with either testosterone propionate (TP), TP plus cyproterone acetate (CA), or oil vehicle for 3 days. DNA synthetic activity was measured through analysis of labelling index (LI) after incorporation of 3[H]-thymidine in vivo. For both UGM + BLE and UGM + Tfm BLE recombinants thymidine incorporation in epithelial cells greatly exceeded that of the stromal cells. TP stimulated epithelial LI to a similar degree (about 50- to 200-fold greater than controls) in both UGM + BLE and UGM + Tfm BLE recombinants; CA antagonized the effect of TP. Nuclear 3H-DHT binding was observed autoradiographically within the epithelial cells of the induced epithelium of UGM + wild-type BLE recombinants, but not within epithelium of UGM + Tfm BLE recombinants. Wild-type mesenchymal cells in both tissue recombinants showed specific nuclear 3H-DHT uptake. Thus, the proliferative effect of androgens upon prostatic epithelium is not a direct effect mediated by intra-epithelial androgen receptors, but rather it appears to be elicited indirectly via regulatory influences from androgen-receptor-positive stromal cells.