The role of GABA in regulating cerebral microvessels was examined in the parenchyma of the hippocampus and the surface of the neocortex. Microvessels were monitored in in vitro slices using computer-assisted videomicroscopy, and synaptically evoked field responses were simultaneously recorded. gamma-Aminobutyric acid (GABA) and the GABAA receptor agonist, muscimol, elicited vasodilation in hippocampal microvessels, whereas the GABAB receptor agonist, baclofen, elicited constriction. The muscimol-induced dilation persisted in the presence of the nitric oxide synthase inhibitor, N-nitro-L-arginine, indicating that this response is not mediated by nitric oxide. Inhibition of neuronal discharge activity with tetrodotoxin did not alter this dilation, but it fully blocked the constrictor response to baclofen. These data suggest that GABAB-mediated, but not GABAA-mediated, responses are dependent on action potential generation. The GABAA receptor antagonists, bicuculline and picrotoxin, elicited constriction, suggesting a tonic dilatory influence by endogenous GABA. Bicuculline-induced constriction was not attenuated by tetrodotoxin. In contrast, these vessels were unresponsive to the GABAB receptor antagonist, 2-hydroxysaclofen. Hippocampal microvessels dilated in response to moderate hypoxia, and this response persisted in the presence of bicuculline, indicating that the hypoxia-induced dilation is not mediated by an action at GABAA receptors. In arterioles located on the surface of the neocortex (i.e., not embedded in the parenchyma of the brain), muscimol elicited vasodilation, whereas bicuculline was ineffective. These results suggest that although these vessels are responsive to GABA, the local concentration of endogenous GABA is insufficient to elicit a tonic effect at rest. These findings raise the possibility that GABA plays a role in local neurovascular signaling in the parenchyma of the brain.