In order to evaluate the effect of dextromethorphan (DEX), an antagonist of N-methyl-D-aspartate receptors, on ischemic damage in the detached retina, transretinal electrical responses (trans-retinal electroretinogram: TR-ERG) to photic stimulation were recorded from the detached retina of rabbit eyes in vitro. In experiment 1, 0.1 ml of 0.1% DEX was injected into the subretinal space to produce retinal detachment (RD) of 8 mm diameter (DEX group). For the control, the same volume of Hanks' solution was injected instead of DEX (control group). In experiment 2, 0.1 ml of 0.1% DEX was dropped onto retina, and RD was produced by injecting 0.1 ml of Hanks' solution into the subretinal space (DEX group). For the control, the same volume of saline was dropped onto the retina instead of DEX (control group). In experiment 1, the averaged b-wave amplitude of the TR-ERG decreased significantly (p < 0.01) at 3 hrs after RD in the control group, but it was almost stable and did not show significant decrease in the DEX group. In experiment 2, the averaged b-wave amplitude of the ERG decreased significantly (p < 0.05) at 2 hrs after the RD in the control, but showed no significant decrease even at 3 hrs after RD in the DEX group. These results indicate that DEX injected into the subretinal space and dropped onto the retina before producing RD may reduce ischemic damage in the detached retina.