The effects of acarbose on cataract development, lens aldose reductase (EC 1·1·1·21) activity and lenticular reduced-glutathione content in diabetic sand rats (Psammomys obesus) were determined. Diabetic sand rats (diet-induced) were fed on diets with or without acarbose(0.4 g/kg) for 39 d. Daily plasma glucose, cataract incidence, aldose reductase and glutathione content were evaluated. After 19 d on acarbose, daily plasma glucose profile was significantly reduced compared with that of sand rats not receiving acarbose. Cataract incidence was markedly lower in sand rats treated with acarbose. After 20 d, cataracts had developed in 90% of the animals fed without acarbose, whereas none was observed in sand rats fed with acarbose. After 37 d acarbose treatment the incidence of cataracts reached only 30%. Compared with untreated animals, lens aldose reductase activity was significantly lower in sand rats fed with acarbose for 39 d (7 6 (SE 0·78) v. 3·5 (SE 0·55) μmol NADPH/mg protein per min respectively, P < 0·001). Concomitantly, significantly higher lenticular protein and reduced-glutathione contents (90 (SE 23) v. 240 (SE 23.5) μg/mg tissue respectively, P < 0·001 and 369 (SE 48·6) v. 645 (SE 71·1)μg/mg tissue respectively, P < 0·001) were found. These results suggest that decreasing hyperglycaemia, accompanied by lower aldose reductase activity obtained by acarbose, led to a significant preventive effect on cataract development in sand rats.