We studied the metabolism of testosterone in primary cultures of prostate epithelial cells and fibroblasts obtained from patients with benign prostatic hyperplasia (BPH). The conversion of 3H-testosterone in both cell cultures was predominantly to the oxidative pathway, with the formation of 3H-androstenedione increasing with cell number and time of incubation. Although we also detected some 5 alpha-reductase activity in these cells, the activity in the stroma component (0.00688 pmol/mg protein/min) was nonetheless insignificant when compared to the 5 alpha-reductase activity in the tissue of origin (0.0616 pmol/mg protein/min) and well below the 17 beta-hydroxysteroid dehydrogenase activity of the same cells (0.0518 pmol/mg protein/min). The aromatase activity in our cells was also measured by two separate techniques, but neither the deuterium procedure nor the production of oestrone from androgen precursors yielded any positive results, suggesting that under these experimental conditions there was no aromatase activity within the cells. The shift from the reductive to the oxidative pathways in these primary cell cultures was reminiscent of the androgen-metabolizing enzyme profiles seen in poorly differentiated prostate cancer. Whether this transition is an obligatory step in the development of hormone refractiveness remains to be elucidated.