Large metastasizing prostate adenocarcinomas (PAs) were induced in L-W rats by a single intravenous (i.v.) inoculation of methylnitrosourea (MNU) and then single subcutaneous implants of testosterone propionate (TP) at intervals of 2 months. This procedure induced PAs in approximately 90% of rats in an average of 11 months. PAs did not develop spontaneously in L-W rats under age 20 months. With this model system, it was demonstrated that within a 14-month observation period, significant numbers of PAs were elicited by TP implants in L-W rats after intervals of 1 week, 1 month and 2 months following the inoculation of MNU. A direct relationship was also demonstrated between amount of TP administered to L-W rats and the resulting incidence of PAs. TP is characterized as a prostate-directed promoter of PA, following a single inoculation of MNU which served as the tumor initiator.