Accumulation of sorbitol has previously been reported in rat mesangial cells cultured under high glucose conditions and has been linked to the impaired function of these cells in streptozotocin-induced diabetic rats. The presence of aldose reductase (AR, EC 220.127.116.11), a key enzyme in the sorbitol pathway, has also been reported in mesangial cells. The role of aldehyde reductase (ALR, EC 18.104.22.168), however, another enzyme catalyzing the conversion of glucose to sorbitol, has yet clarified to be in mesangial cells. In the present study, an aldose reductase inhibitor, FK-366, was capable of significantly inhibiting DL-glyceraldehyde (GAD)-reducing activity in a concentration of 10-8 M, but was unable to inhibit D-glucuronate (GLN)-reducing activity in cultured rat mesangial cells, even at 10-5M confirming the highly specific inhibitory activity of this compound on AR and suggesting simultaneous presence of both enzymes, AR and ALR. FK-366 also significantly prevented the accumulation of sorbitol in mesangial cells cultured under high glucose (55 mM) conditions 10-7M, the concentration at which the compound inhibited GAD-reducing activity but not GLN-reducing activity. These results suggest that AR may play a major role in sorbitol accumulation in cultured rat mesangial cells under high glucose conditions, although sorbitol may be produced by other enzymes.