Excitatory postsynaptic field potentials (termed U1 and U2 waves) were extracellularly recorded from superficial layers of the frog optic tectum in vitro following stimulation of the contralateral optic nerve. gamma-Aminobutyric acid (GABA) largely enhanced the U1 and U2 waves, an effect which was progressively diminished by lowering the extracellular Cl- content. Muscimol or 3-aminopropanesulphonic acid (3-APS) was more potent than GABA in enhancing the U1 and U2 waveforms, whereas trans-4-aminocrotonic acid (TACA) was equipotent, its cis isomer (CACA) being less active. The action of GABA, TACA and CACA was relatively insensitive to bicuculline (100 microM). Pentobarbitone increased the U1 and U2 waves and antagonized the action of GABA. Midazolam did not change the GABA-induced responses. Picrotoxin antagonized both pentobarbitone and GABA effects. It is suggested that in the frog optic tectum GABA can activate an atypical receptor mechanism blocked by picrotoxin but comparatively insensitive to bicuculline.