Effects of gamma-aminobutyric acid (GABA) and some selective GABAergic ligands on the quantal acetylcholine (ACh) release in the frog neuromuscular contacts were investigated using combination of microelectrode technique with fluorescent and immunohistochemical assays. Significant attenuation of ACh release was observed in the presence of GABA as well as selective GABA and GABA receptor agonists. Neither GABA nor GABA antagonists abolished to full extent this effect of GABA. Fluorescent assay allowed to detect the GABA-induced opening of K channels, which was inhibited by XE-991, a selective antagonist of K7 type. Electrophysiological recordings of endplate potentials in the presence of XE-991 confirmed the contribution of K7 type potassium channels to the effects of GABA on ACh release that was not associated with GABA and GABA receptors activation. Note that XE-991 effectively precluded the action of retigabine, neuronal K7 channel opener, on ACh release. Immunohistochemical assay revealed that frog mature skeletal muscle fibers contain a significant amount of GABA, and substantial amount of GABA can be released in the extracellular space at the muscle contractions induced by prolonged high-frequency nerve stimulation. Besides, some binding sites for exogenous GABA were detected on the plasma membranes. It is concluded that GABA, in addition to affecting GABA and GABA receptors, can directly activate K7 channels, thereby negatively modulating the evoked ACh release. Endogenous GABA may serve as a retrograde regulator of neurotransmitter exocytosis.