Anterior, dorsal, lateral, and ventral prostate lobe transplants derived from 12-week-old donor F<sub>344</sub> rats were maintained in the cleared 5th mammary fat pads of 3-week-old testosterone propionate (TP)-implanted female and TP-implanted and unimplanted male syngeneic hosts for 12 posttransplantation weeks. Prostate transplants failed to survive in untreated female hosts. Implantation of TP pellets in male hosts resulted in an increase in the number of surviving transplants as well as in the ability of these transplants to biotransform testosterone-<sup>3</sup>H (T-<sup>3</sup>H) to its major radiometabolites (dihydrotestosterone-<sup>3</sup>H and androstanediol-<sup>3</sup>H) in vitro. TP implants + prolactin (50IU/kg daily i.p. injections for the 5 days prior to sacrifice) caused a marked increase in prostate weight and in epithelial secretory activity. Whereas the anterior, dorsal, and ventral lobe transplant showed acini with normal columnar epithelium and secretory activity, the epithelial cells in the lateral lobe transplants were often very irregular. In general, those prostate transplants which showed an ability to metabolize testosterone were also those exhibiting normal secretory epithelium.