Embryonic urogenital sinuses (UGS) of 16-day-old mice were divided into two or three zones (cranial half and caudal half; or cranial third, intermediate, and caudal thirds). Following tryptic digestion, these zones were separated into their mesenchymal (UGM) and epithelial (UGE) components. The cranial and caudal zones of the UGM were recombined separately with the different zones of the UGE or with adult mouse urinary bladder epithelium (BLE). Following four weeks of growth in syngeneic male hosts, tissue recombinants of cranial UGM + caudal UGE contained numerous prostatic ducts and urethral glands. Conversely, recombinants composed of caudal UGM + cranial UGE developed into fibromuscular tissue covered with urethral epithelium and did not contain prostate, but occasionally contained urethral glands. Urethral glands were usually present in grafts of cranial UGM + cranial UGE or in grafts of the intact intermediate third of the UGS. In heterotypic recombinants of the UGM + adult BLE, prostatic glands were induced only when cranial UGM was utilized. Urethral glands were not observed in tissue recombinants prepared with adult BLE. These data suggest that regional differences in mesenchymal inductive ability and epithelial responsiveness play a role in the harmonious morphogenesis of the male lower genitourinary tract.