The androgen dependent prostatic carcinoma of human origin, PC-82, was used as a model system to investigate the effect of various levels of androgen on the growth of prostatic tumor tissue. Plasma testosterone levels in mice were correlated to tumor growth and intratumor concentrations of testosterone and 5 alpha-dihydrotestosterone. PC-82 tumor burden remained stable at plasma testosterone levels of 0.8 nmol/l., whereas tumor growth occurred at higher levels and tumor regression was observed at lower plasma levels. This critical level of testosterone corresponded with intratumor testosterone and 5 alpha-dihydrotestosterone concentrations of six to 10 and three to four pmol/gm. tissue, respectively, which are significantly above the levels found in castrated non-supplemented animals (3.1 and 1.4 pmol/gm. respectively). This indicates that remaining concentrations of dihydrotestosterone, which amount to two to three times the castrate level, are not stimulatory for tumor growth in the model of the androgen dependent PC-82 tumor.