Catfish (Ictalurus punctatus) cone horizontal cells contain NMDA receptors whose function has yet to be determined. In the present study, we have examined the effect of NMDA receptor activation on voltage-gated ion channel activity. NMDA receptor activation produced a long-term down regulation of voltage-gated sodium and calcium currents, but had no effect on the delayed rectifying potassium current. NMDA's effect was eliminated in the presence of AP-7. To determine if NMDA receptor activation had functional implications, isolated catfish cone horizontal cells were current clamped to mimic the cell's physiological response. When horizontal cells were depolarized, they elicited a single depolarizing overshoot and maintained a depolarized steady state membrane potential. NMDA reduced the amplitude of the depolarizing overshoot and increased the depolarized steady state membrane potential. Both effects of NMDA were eliminated in the presence of AP-7. These results support the hypothesis that activation of NMDA receptors in catfish horizontal cells may affect the type of visual information conveyed through the distal retina.