Genistein is a phytoestrogen and is found in many plants consumed by humans and animals. This isoflavone was found to exert metabolic effects, especially on lipid and carbohydrate metabolism. The aim of this experiment was to determine whether genistein at a dose of 1 and 5 mg/kg body weight administered intragastrically to male and female adult rats changes insulin, leptin, thyroid hormone, and metabolic parameters. The results suggest that genistein has only a slight influence on metabolism. A substantial reduction of triglyceride stores was observed in the skeletal muscles. This effect was sex-dependent and occurred only in females. Moreover, it was demonstrated that genistein at the higher dose decreased blood insulin and leptin levels.