Glutamate is a very abundant amino acid in the retina. Unless the concentration of glutamate in subretinal space is quite accurately regulated, the excitotoxic effects of this amino acid may appear. Tamoxifen is widely used in the therapy of breast cancer and has been reported to cause ocular toxicity. We investigated the effect of tamoxifen in vitro on retinal pigment epithelium (RPE). RPE is a monolayer of cells between choroidal capillaries and photoreceptors. The problem was if one of the mechanisms of tamoxifen's ocular toxicity could be the change in glutamate uptake in RPE. RPE cells from pig eyes were used to set a cell culture. In measuring the glutamate uptake, [ 3 H]L-glutamate and secondary culture were used. The total concentration of glutamate was 5 μM, and the incubation time was 10 minutes. Tamoxifen was tested with 6 different concentrations. Glutamate uptake was reduced by the presence of tamoxifen in vitro. The reduction increased with growing tamoxifen concentration. The results suggest that the antiestrogenic drug tamoxifen affects the uptake of glutamate in retinal pigment epithelium. By reducing the amount of glutamate entering RPE cells, tamofixen possibly enhances the accumulation of glutamate in subretinal space. The high concentration of glutamate in subretinal space could disturb retinal functions and cause retinal toxicity by excitotoxic mechanisms.