In order to assess the androgenic potency of physiological plasma concentrations of the adrenal steroids dehydroepiandrosterone (DHEA) and androstenedione (delta 4-dione) in the rat prostate, these two steroids were released from Silastic tubings of appropriate length and size in castrated male rats. Implants of DHEA led to plasma levels of DHEA and 5-androsten-3 beta,17 beta-diol covering the range of concentrations found in adult men while no significant change was observed in plasma levels of delta 4-dione, testosterone and dihydrotestosterone (DHT). delta 4-Dione implants, on the other hand, led to a parallel increase in plasma delta 4-dione and testosterone levels at all doses used while plasma DHT only increased at supraphysiological doses of delta 4-dione. At plasma concentrations comparable to those found in adult men; delta 4-dione (0.8 ng/ml) and DHEA (3.4 ng/ml) stimulated prostate weight 3.7- and 2.1-fold, respectively. In the same groups, prostatic DHT levels were elevated at 4.48 +/- 0.05 and 2.70 +/- 0.73 ng/g tissue, respectively. A close parallelism was observed between prostatic DHT levels and prostatic weight in all groups. The present data show that in the rat, a species having no significant secretion of adrenal androgens, plasma concentrations of DHEA and delta 4-dione maintained within the range of those found in adult men are efficiently converted into DHT and act as potent androgenic stimuli in prostatic tissue. The castrated rat bearing Silastic implants releasing constant and predetermined amounts of adrenal steroids offers a good model to study the recently identified role of adrenal steroids in peripheral tissues.