The effect of chronic treatment of female guinea-pigs with dihydrotestosterone (DHT) on growth and function of the adrenal gland and, in particular, on the reticular zone is described. Two groups of 6 young adult, female guinea-pigs were treated with DHT (1 mg/kg dissolved in peanut oil and injected s.c.) for 30 and 60 days. Two other groups of animals, treated only with oil, were used as controls. At the end of treatment, animals were killed and adrenal glands were quickly removed. Plasma levels of pregnenolone, dehydroepiandrosterone (DHA) and its sulfate (DHA-S), 17 alpha-hydroxyprogesterone, androstenedione, testosterone, estradiol, 11-deoxycortisol, androstenedione, DHT and 3 alpha-androstanediol were determined by R.I.A. following celite microcolumn chromatography. Animals treated for 30 days showed only elevated DHT and 3 alpha-androstanediol plasma levels, whereas animals treated for 60 days also showed increased values of pregnenolone (251 +/- 62 vs 193 +/- 51 ng/dl; P less than 0.05), DHA-S (12,046 +/- 4110 vs 2780 +/- 888 ng/dl; P less than 0.001) and slightly increased values of DHA (110 +/- 31 vs 86.5 +/- 55.4). In the 30-day-treated animals no histological changes were observed, but in the 60-day-treated group the total size as well as cell volumes of the zona reticularis were significantly increased. Normal estrous cycles were observed in the 30-day-treated animals whereas the 60-day-treated animals showed a progressive acyclicity during the second month of treatment. These results indicate that in guinea-pigs, prolonged treatment with DHT induces a growth of the zona reticularis of the adrenal gland associated with increased levels of 5-ene steroids, particularly DHA-S. The mechanisms inducing these modifications are probably mediated by a DHT effect at the hypothalamic-pituitary level. A direct effect of DHT on the zona reticularis, however, cannot be excluded.