The effects of gamma-aminobutyric acid (GABA) on neurones of the amphibian optic tectum were studied with current- and voltage-clamp recording from an isolated preparation of the midbrain of the frog Rana temporaria. Bath-applied GABA (1 mM) enhanced depolarizing synaptic potentials evoked in layer 6 tectal neurones by orthodromic stimulation of the optic tract. GABA also facilitated Na(+)- and Ca(2+)-dependent action potentials elicited by intracellular injection of depolarizing current. These actions of GABA were associated with comparatively small changes in membrane potential and their reversal potential was dependent on the Cl- equilibrium potential. Changes in input resistance observed during application of GABA were small and in part accountable for by the rectifying properties of the cell membrane. Tetrodotoxin (TTX; 1 microM) did not block the action of GABA on these neurones. These results show that externally applied GABA was able to raise directly the intrinsic excitability of frog tectal neurones and to enhance excitatory synaptic transmission elicited by stimulation of optic nerve fibres.