Permanent transplantable human tumors in nude mice offer the possibility of studying the impact of different (endocrine) treatment regimens on human cancer tissue. Among the limited number of human tumor models in athymic nude mice developed so far, the PC-82 tumor (which was established in our institution) mimics many of the important properties of clinical prostate cancer. The absence of PC-82 tumor growth in intact female and in castrated male mice indicates the absolute requirement of androgen for the growth of this tumor. Delayed testosterone (T) substitution (up to 70 days after tumor grafting) in PC-82 transplanted female mice resulted in tumor growth. This indicated that after androgen withdrawal at least part of the cells do not die and keep the capability to respond to androgens. Androgen withdrawal from T-implanted tumor-bearing female mice caused a rapid reduction (90% within 1 day) of the tissue T and a slower decline (up to 90% within 7-10 days) of tissue DHT concentrations. By the use of Silastic implants, containing different proportions of T mixed with cholesterol, circulating T levels of 0.2-20 nmol/L were obtained. It was observed that a constant plasma T level between 1 and 2 nmol/L (achieved with 10% T implants) is the threshold below which growth of the PC-82 tumor tissue is no longer stimulated.