Androgen-mediated growth repression of androgen-independent prostate cancer (AIPC) cells has been reported in androgen-independent PC-3 cells overexpressing the androgen receptor, and in androgen-independent derivatives of LNCaP cells that develop following prolonged culture in androgen-free media. Using two models of AIPC, PC3/AR cells and LNCaP-OM1 cells, a subclone of LNCaP cells derived by prolonged culturing in charcoal-stripped media, we investigated whether expression of neutral endopeptidase 24.11 (NEP), a cell-surface peptidase that cleaves and inactivates neuropeptides implicated in the growth of AIPC, is induced by androgen, and whether NEP contributes to the observed androgen-mediated growth repression. These cell lines each express high levels of androgen receptor. Culturing in dihyrotestosterone (DHT) resulted in a 30-56% (PC3) and 35-43% (LNCaP-OM1) decrease in cell number over 7 days concomitant with a significant increase in NEP enzyme specific activity. Northern analysis detected an increase in NEP transcripts following DHT treatment in PC3/AR cells. The addition of the NEP enzyme inhibitor phosphoramidon to PC3 and LNCaP-OM1 or the NEP competitive inhibitor CGS 24592 to LNCaP-OM1 blocked the increase in NEP enzyme activity and reversed the DHT-induced growth inhibition. Neither phosphoramidon or CGS 24592 alone inhibited cell growth. Furthermore, the reversal of growth inhibition in LNCaPOM1 cells was dose dependent on the concentration of CGS 24592. These data indicate that androgen-induced growth repression of AIPC cells PC3 and LNCaP-OM1 results in part from androgen-induced expression of NEP in these cells.