The objective of the present study was to determine if a specific window of development (neonatal/ prepubertalonly, adult only, or life-time) is effective for genistein chemoprevention of prostate cancer, and the potentialmechanisms of genistein chemoprevention in vivo. Male Lobund-Wistar (L-W) rats were fed zero or 250 mggenistein/kg AIN-76A diet at designated periods of time and then injected with N-methylnitrosourea (NMU)into the dorsolateral prostate (DLP) on day 70 for cancer initiation. Rats were necropsied at 11 months. Theincidence of poorly differentiated (PD) carcinomas was 43.5% in rats fed a phytoestrogen-free AIN-76A dietonly, 29.6% in rats provided genistein in the diet from postnatal days 1-35, 28.6% in rats fed genistein frommonths 3-11, and 20% in rats provided genistein from birth through 11 months. Genistein induces cell apoptosisand inhibits cell proliferation in both prostate cancerous and nontumorigenic DLP. These actions are accompaniedwith the regulation of PTEN/Akt-AR axis. Our data demonstrate that dietary genistein reduces the incidence ofadvanced prostate cancer induced by NMU in L-W rats during adult and life-time exposure, the latter beingmore effective. The regulation of AR/Akt/PTEN axis by genistein may be one of the molecular mechanisms bywhich it inhibits cell proliferation and induces apoptosis, thus providing evidence of roles of genistein in prostatecancer prevention and treatment.