Abstract Effects of agonists of the excitatory amino acid (EAA) transmitters were examined in carp cone horizontal cells where glutamate (Glu) or aspartate (Asp) is believed to act as the transmitter released from the photoreceptors. Bath application of kainic (KA), quisqualic (QA) and N- methyl- d-aspartic (NMDA) acids produced little effect on cone cells, indicating that their effects act directly on the horizontal cells. KA and QA (100 μM for both) produced depolarizations in the horizontal cells. Their reversal potentials were measured by our novel technique 16 which was developed to overcome a serious experimental disadvantage due to electrical coupling between horizontal cells. The retina was perfused with a modified Ringer solution which contained high-Ca 2+,Ba 2+, and some K +-channel blockers. A Ca 2+ action potential having an overshoot was evoked in the horizontal cells when they were depolarized by application of the EAA. During the action potential, perfect potential uniformity was achieved throughout electrically coupled cells. Responses induced by KA and QA during the overshoot appeared in reversed polarities to those elicited at the resting state. Their reversal potentials were then estimated to be similar at around −6mV, and this value coincided with that of the Glu-or Asp-induced responses. On the other hand, effects of NMDA were diverse even though applied in the order of mM; some cells were hyperpolarized, but the others were little affected. These observations indicate that the EAA receptor of carp horizontal cells is KA/QA (non-NMDA) type.