1. We studied the effect of zinc on the gamma-aminobutyric acid-C (GABAC) receptor in acutely isolated catfish cone horizontal cells using the whole cell patch-clamp technique. 2. GABA activates the GABAC receptor with a half-activation concentration (EC50) of 2.99 microM. The Hill coefficient is 1.32. Desensitization of the receptor is evident when GABA concentration is > 3 microM. 3. Zinc downmodulates the GABAc receptor current, elicited by 30 microM GABA, with a half-inhibition concentration (IC50) of 8.20 microM. 4. The inhibition of zinc is both competitive and noncompetitive. In the presence of 10 microM zinc, the maximum GABA response was reduced to approximately 60 percent of control and the EC50 increased to 17.32 microM, whereas the Hill coefficient (1.39) was not significantly altered. 5. The steady-state block by zinc is virtually voltage independent. 6. These results suggest that the GABAC receptor of horizontal cells can be modulated by endogenous zinc found in photoreceptors.