1. Whole-cell patch-clamp recordings were obtained from ON bipolar cells in a retinal slice preparation of the mudpuppy, Necturus maculosus. The effects of excitatory amino acid (EAA) agonists applied in the presence of cobalt (2-5 mM) were examined. 2. At the holding potential of -50 mV, L-2- amino-4-phosphonobutanoic acid (L-AP4, 5-10 μM) evoked an outward current accompanied by a conductance decrease. The zero current potential of the L- AP4-evoked current was near 0 mV independent of whether the intracellular Ringer solution contained CsCl or CsCH3SO4. The currents evoked by light were also accompanied by a conductance decrease and reversed near 0 mV. Replacing external sodium with choline or N-methyl-D-glucamine generated an outward current and suppressed the response to L-AP4. The response to L-AP4 was enhanced by removing extracellular calcium and suppressed by increasing extracellular calcium. These results indicate that L-AP4 closes nonspecific cation channels that are blocked by extracellular calcium. 3. In 2 mM cobalt, α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA, 50-100 μM) evoked membrane currents that were accompanied by a conductance increase. AMPA-evoked currents exhibited a significant chloride dependence and were suppressed by γ-aminobutyric acid-A (GABA(A)) antagonists bicuculline and picrotoxin; a GABA uptake blocker, nipecotic acid; and a glycine antagonist, strychnine. AMPA-induced currents were virtually absent in the presence of 5 mM cobalt and nominally 0 mM extracellular calcium. These results indicate that the conductance increase induced by AMPA in the presence of 2 mM cobalt is largely the result of calcium-dependent synaptic inputs onto GABA(A) and glycine receptors of ON bipolar cells. 4. N-methyl-D-aspartic acid (250 μM) was ineffective when applied in the presence of 100 μM cadmium or 2 mM cobalt. 5. 1S,3R/1R,3S-1-aminocyclopentane-1,3-dicarboxylic acid (100-200 μM) evoked an outward current accompanied by a conductance decrease and appears to be an agonist at the L-AP4 receptor. 6. The findings of this study suggest that the only type of EAA receptor in mudpuppy ON bipolar cells is the L-AP4 receptor and that L-AP4 receptor activation results in the closing of nonspecific cation channels that are blocked by extracellular calcium.