The mechansim of γ -aminobutyric acid (GABA)-induced suppression of 5-hydroxytryptamine (5HT)-induced contractility of cerebral blood vessels was studied in single smooth muscle cells isolated from the guinea-pig basilar artery. GABA reduced 5HT-induced contraction of single smooth muscle cells, and the effect of GABA was mimicked by muscimol, but not baclofen. The response of muscimol was antagonized by bicuculline, thereby indicating that GABA<sub>A</sub> receptors exist on the smooth muscle of the basilar artery. Since GABA did not change the contraction induced by the addition of Ca<sup>2+</sup> to the Ca<sup>2+</sup>-free medium in the presence of high K<sup>+</sup>, it is unlikely that GABA inhibits the influx of extracellular Ca<sup>2+</sup>. The caffeine-induced contraction in the Ca<sup>2+</sup>-free medium was reduced by GABA, and the effect of GABA was not obtained by treatment with furosemide and in the Cl<sup>-</sup>-free medium. These results indicate that GABA acts on the GABA<sub>a</sub> receptor located on smooth muscle cells and reduces the contractility of the basilar artery by suppression of the mobilization of intracellular Ca<sup>2+</sup>.