The NB rat prostate adenocarcinoma model is one in which microscopic carcinoma can be induced via the use of testosterone and estrogen silastic implants. There is an incidence of large, grossly-positive tumors; which approaches 10% to 15%. Seventy-three percent of the animals subjected to this induction method for periods of 9 to 18 months developed at least microscopic carcinoma. The earliest appearance microscopic foci of carcinoma was after three months of implantation, in which one of 30 animals so treated did show carcinoma. However, 88% of the animals treated for 18 months or longer showed microscopic carcinoma.