The polyol pathway was investigated in primary cultures of human retinal pigment epithelial (HRPE) cells and the results were compared with those in human lens epithelial (HLE) cells cultured under similar conditions. Significant levels of galactitol were formed in HRPE cells cultured for 72 hr in a medium containing 30 mmol/l D-galactose. Polyol accumulation was accompanied by the appearance of vacuoles as seen by transmission electron microscopy (TEM). Biochemical analysis revealed a significant depletion of cellular myoinositol, taurine, and a number of other free amino acids similar to those in HLE cells. These morphologic and biochemical changes observed in HRPE cells cultured in high galactose medium were inhibited or prevented by the inclusion of an aldose reductase inhibitor in the medium, further supporting the view that vacuole formation is due to the osmotic effect of polyol formation mediated by aldose reductase. The similarity of intracellular vacuole formation resulting from polyol accumulation and the biochemical changes in HRPE and HLE cells strongly suggests that a common mechanism is involved.