A simple and selective method for genistein (GNST) determination in rat liver and kidney was validated in order to study the phytoestrogenic effect of GNST in ovariectomised female Wistar rats. GNST was separated on a Kromasil 100-RP8 column, 150 mm x 4.6 mm, 5 mm equipped with a Kromasil RP 8 precolumn. The mobile phase was 55:45 (v / v) phosphoric acid, 15 mmol in water: methanol at a flow rate of 1.3 ml / min. Luteolin 20 μg / ml in methanol was used as internal standard (IS). The retention time of GNST was tR = 13.22 min and tR =11.60 min for the IS. Calibration curves in the range 40-400 μg GNST/100g liver and 20-200 μg GNST/100g kidney presented a coefficient of determination higher than 0.99. The method developed presented a good precision and accuracy at the lower limit of quantification LLOQ. 10 white Wistar female rats, 8 weeks of age were treated s.c. with 10 mg GNST/kg bw/day for 8 weeks, while a group of 10 animals were used as controls. The values obtained for GNST in the liver were 192.12 ± 53.46 μg/100g and 74.51 ± 12.77 μg/100g in kidney samples.