We have investigated the effect of free radicals on the electrical gap junctions between horizontal cells in the carp retina. In our previous study, L-buthionine sulfoximine, an inhibitor of glutathione synthesis, caused uncoupling of horizontal cells four days after injection. In the present study, we have used paraquat, a generator of exogenous reactive oxygen species, to investigate whether it was the depletion of glutathione or an increase in the level of reactive oxygen species which resulted in horizontal cell uncoupling after L-buthionine sulfoximine injection. Intracellular recordings were made from L-type horizontal cells at various time-points after intravitreal injection of paraquat. Injection of 25nmol paraquat caused an increase in response amplitude to central spot light stimuli by two days after injection, which continued for a further two to three days and had almost disappeared by seven days after injection. There was also a sharp increase in reactive oxygen species production, peaking at four days and disappearing by seven days after injection, and an accompanying depletion and a restoration of glutathione levels with a similar time-course. Marking cells with Lucifer Yellow clearly illustrated uncoupling of horizontal cells after paraquat injection. If paraquat and L-buthionine sulfoximine were injected simultaneously, the increase in response to central spots was observed as early as one day after injection. This response amplitude was not more enhanced than that observed after L-buthionine sulfoximine injection alone, although a dramatic increase in the level of reactive oxygen species was observed. From these results, we suggest that reactive oxygen species are involved in uncoupling, while recovery from uncoupling is dependent on glutathione. Furthermore, we conclude that a balance between glutathione and reactive oxygen species levels is the most important factor controlling gap junctional intercellular communication of L-type horizontal cells in the carp retina.