The water extract of Aralia elata (Aralia extract) has been used in Korean traditional medicine to treat diabetes mellitus. Here, we investigated the aldose reductase inhibitory activity, antioxidant activity and anticataract capacity of Aralia extract using various experimental systems. To determine its aldose reductase inhibitory activity and its antioxidant effect, we used rat lens homogenates. Rat lens cultures and a rat model were used to observe anticataract activity. The resulting IC50 value of Aralia extract in vitro as an aldose reductase inhibitor was 11.3 microg/ml and as an antioxidant was 25.1 microg/ml. Rat lenses in media containing 20 mM xylose developed a distinctly opaque ring in 9h, and treatment with Aralia extract at a concentration of 1mg/ml lowered lens opacity by 36.4 and 31.3% after 24 and 48 h, respectively. In vivo experiments were performed with streptozotocin (STZ) induced diabetic rats. The diabetic control animals developed cataracts at 11 weeks after STZ injection, while oral Aralia extract administered at 300 and 600 mg/kg body weight for 11 weeks reduced cataract formation by 15 and 12%, respectively. The present study shows that Aralia extract inhibits aldose reductase and acts in vitro as an antioxidant, and suggests that these activities have a preventive effect on cataractogenesis in xylose containing lens organ cultures and in in vivo in STZ induced diabetic rats.