The phytoestrogen, 8-prenylnaringenin (8-PN), is the most potent phytoestrogen discovered to date. Derived from hops, it is present in dietary supplements currently marketed for natural breast enhancement, though little is known about efficacy rates or other effects (Coldham & Sauer, 2001). 8-PN is also of potential interest in the treatment of menopausal symptoms and diseases involving angiogenesis. It is known that various phytoestrogens produce inhibitory effects on gonadotrophin secretion in both humans and animals (McGarvey et al, 2001). The aims of the present study were to investigate effects of 8-PN on the reproductive axis, both in vivo, by examining effects of 8-PN on LH pulses, and in vitro, through studying the effects of 8-PN on GnRH mRNA expression in the GnRH cell line, the GT1-7 cells. Ovariectomized (ketamine, 100 mg kg^-1, ip) Wistar rats were chronically implanted with intravenous (iv) cannulae (ketamine, 100 mg kg^-1, ip). Blood samples (25μl) were collected at 5 min intervals for 8hrs for the detection of LH. After 2hr sampling either 17β-oestradiol (E2, 0.2μg bolus followed by 0.2μg/h for 6h) or 8-PN (20μg bolus followed by 20μg/h for 6h) were infused intravenously (n=7). Thirty minutes prior to the end of sampling GnRH was administered (500ng/kg, iv bolus injection) to test pituitary responsiveness. GT1-7 cells were grown in Dulbecco’s modified eagle medium supplemented with 10% fetal calf serum, 4.5 mg/ml glucose and antibiotics and maintained at 37°C. GT1-7 cells were cultured with 8-PN concentrations ranging from 10^-5M to 10^-8M, then harvested on ice and total RNA was extracted. GnRH mRNA levels were investigated by quantitative real time RT-PCR. Both E2 and 8-PN, caused a profound reduction in LH pulse frequency (68.0±7.3% and 148.7±18.1% increases in LH inter-pulse interval, respectively, mean±sem) and amplitude (33.6±4.3% and 47.4±6.7% decrease in LH pulse amplitude respectively) compared with controls (p<0.05). Furthermore 8-PN blocked pituitary response to GnRH. In the GT1-7 cells, 8-PN treatments induced a dose dependent suppression in GnRH mRNA expression (63.9±7.2% at 10^-5M 8-PN). These results suggest that suppression of pulsatile LH secretion by 8-PN may involve a hypothalamic and pituitary site of action. Further a direct inhibitory action at the level of the GnRH neurone is also implicated.