The influence of isoflavone-genistein on some hormones and metabolic parameters in sexually immature, female rats was studied. Rats (34 days old) were divided into three groups: control (receiving no genistein), receiving 1 or 5 mg of genistein/kg body weight (BW). The tested compound was administered intragastrically, i.e. by using a cannula inserted via oesophagus into the stomach once a day for seven days. In genistein-treated rats body weight gains were unchanged in comparison to control animals. Genistein at a higher dose caused a significant (P≤0.01) decrease in blood insulin concentrations. Simultaneously, in animals treated with this phytoestrogen (5 mg/kg BW) blood leptin was significantly reduced (P≤0.01). This latter effect was probably due to the direct inhibitory influence of genistein on leptin secretion from adipocytes. Thyrotropin, total and free triiodothyronine and thyroxine were not changed by the tested phytoestrogen. Several metabolic parameters determined in blood (glucose, triglycerides, free fatty acids, cholesterol), liver (glycogen, triglycerides and cholesterol) and muscle (glycogen and cholesterol) were also unaltered in rats receiving genistein. However, genistein administered at a higher dose evoked a significant reduction (P≤0.01) of muscle triglyceride stores. Results obtained in the experiment indicate that genistein may affect insulin and leptin-hormones responsible for metabolism, energy expenditure and food intake in young, sexually immature female rats. The muscle triglicerydes are the only metabolic parameter affected by the compound in these rats.