The proliferative responses of rat prostate and human benign prostatic hyperplasia have been followed in organ culture using [125I] iododeoxyuridine uptake to monitor DNA synthesis. In serum-free cultures, testosterone induced a marked increase in DNA synthesis (three-fold) in 4- to 6-month-old rat prostates at concentrations of 4 x 10(-9) to 4 x 10(-6) M, whereas in greater than 12-month-old rat prostates the response was less marked. Human benign prostatic hyperplasia also showed an increased uptake at similar testosterone concentrations and of a similar magnitude to the response of greater than 12-month-old rat prostates. At 10(-5) M DNA synthesis was markedly suppressed in cultures of both rat and human prostate. The proliferative response of human benign prostatic hyperplasia increases up to days 3 to 4 in culture and then declines in both control and hormone-treated groups and may represent repair processes which appear to be hormone dependent.