Zinc has been implicated in steroid endocrinology of the prostate gland; and 5 alpha-dihydrotestosterone (DHT) is believed to express androgenic responses in the prostate. To note the effect of neutralized zinc (zinc gluconate + arginine) on the prostate, 50 sexually mature rats, weighing 325 +/- 20 g, were divided into five groups as follows: (1) control, (2) sham, (3) castrated, and injected intraprostatically with (4) 10 mg neutralized zinc, and (5) 20 mg neutralized zinc. Results indicated significant reduction (P < 0.05) of prostate weight, 5 alpha-reductase activity, and total protein and DNA concentrations in treated prostate tissue; no significant change in weight and histological structure of testes, epididymides, and seminal vesicles; and no significant effect on progeny and blood testosterone level of treated animals. These results suggest that direct application of neutralized zinc to the prostate offers a new modality for treatment of prostatitis without affecting spermatogenesis.