Both active transport through and permeability of the blood-retinal barrier (BRB) are affected by sodium iodate, while the blood-brain barrier (BBB) is more resistant. We studied the effect of sodium iodate on facilitated diffusion through the two barriers. The retinal (RUI) and brain (BUI) uptake indices were determined for D-glucose and two neutral amino acids in normal and sodium iodate-treated rats. The integrity of the barriers was estimated by RUI and BUI for L-glucose and by measuring tissue uptake of L-glucose after an intravenous injection. We found that 30 mg/kg sodium iodate had no effect on transport through or permeability of the BBB, while 20 mg/kg significantly (P < 0.02) reduced transport of D-glucose, but not amino acids, through the BRB 1 h after injection. After 24 h both 20 mg/kg and 30 mg/kg sodium iodate caused a significant disruption of the BRB (P < 0.005 and P < 0.001, respectively). Thus, sodium iodate selectively affects the carrier for D-glucose in the BRB but not in the BBB. The presence of an epithelial part in the BRB, the retinal pigment epithelium, may explain the difference between the two barriers.