Solitary horizontal cells isolated from goldfish retinae are depolarized by L-glutamate (Glu) (Ishida, Kaneko & Tachibana, 1984), a possible candidate for the transmitter of photoreceptors. The underlying mechanisms were analysed under voltage-clamp conditions using 'giga-seal' suction pipettes in the whole-cell recording configuration. Glu induced an inward current at the resting membrane potential (ca. -57 mV). Membrane depolarization decreased the amplitude of Glu-induced current and reversed its polarity to outward beyond approximately -3 mV. Membrane hyperpolarization below the resting potential decreased the amplitude of the Glu-induced inward current. When a K current through the anomalous rectifier, which is activated by membrane hyperpolarization (Tachibana, 1983), was blocked by Cs ions, this phenomenon disappeared and the Glu-induced current increased in amplitude with hyperpolarization. Mg ions had no effect on the reduction of the Glu-induced current at hyperpolarized potentials. It was strongly suggested that Glu produced two types of conductance change; a conductance increase due to an activation of Glu channels and a conductance decrease due to a blockage of the K current through the anomalous rectifier. The latter effect is analysed in detail in the following paper (Kaneko & Tachibana, 1985b). The Glu-activated channel was permeable to cations (Na, K, Ca, Mg, Tris and choline ions) with low selectivity, but not to anions. The least effective dose of Glu was less than 10 microM. The relation between the Glu-induced current and the membrane potential curved upwards near the reversal potential, and this relation was not affected by Mg ions.