Pellets of crystalline testosterone or testosterone mixed with cholesterol in ratios of 1:2 or 1:5 were implanted in one lobe of the ventral prostate of castrated Sprague-Dawley rats. The weights and histological appearance of these lobes when compared to those of the contralateral prostatic lobes provide evidence for a greater stimulation or maintenance of growth of the lobes containing the androgen pellets. Inert pellets such as cholesterol had no such effect. This demonstration of a correlation between the presence of testosterone locally and the rate of growth of a lobe of the ventral prostate provides evidence for a local action of testosterone on the prostate. The technique employed in this study should be useful in a search for, and an evaluation of, substances directly inhibiting the action of androgen on the prostate.