In an attempt to identify the mechanisms underlying the ill effects of high glucose previously described in cultured human endothelial cells, we have investigated in these cells the activity of the polyol pathway and accumulation of glucose 6-phosphate, a powerful agent of non-enzymatic glycosylation. Sorbitol accumulation varied among different batches of cells (primary cultures). In presence of 5 mmol/l glucose the cellular sorbitol content ranged from 0.04 to 0.12 nmol/10(6) cells. When cells were exposed to 20 mmol/l glucose the sorbitol content increased by 2- to 3-fold to concentrations of 0.08-0.38 nmol/10(6) cells (p less than 0.01). Addition to the culture medium of 100 mumol/l Sorbinil, an inhibitor of aldose reductase, resulted in a substantial inhibition of sorbitol accumulation throughout the 14 days in culture, but the degree of inhibition varied inversely with the duration of cell exposure to high glucose (70% inhibition in cells exposed to high glucose and Sorbinil for 1-3 days versus 14% inhibition in cells exposed for 14 days, p less than 0.01). Sorbinil treatment failed to improve even slightly the abnormalities in cellular replication induced by high glucose. The cellular content of glucose 6-phosphate was augmented 3-fold by exposure to 20 mmol/l glucose (p less than 0.001). In conjunction with other studies these results indicate that in this model the polyol pathway is not an osmotically or metabolically important mechanism of glucotoxicity, and that the inhibitory activity of Sorbinil on the polyol pathway of human tissues may be a function of their length of exposure to hyperglycaemia. The consequences of intracellular accumulation of glucose 6-phosphate await investigations.