Although the neurophysiological and neurotrophic actions of GABA in the retina are well-recognised, less is known about the role of GABA in the regulation of the retinal microcirculation. In the present study, we have examined the actions of GABA on retinal arterioles of the rat retina using ex vivo retinal whole mount preparations and isolated vessel segments. All animal experiments conformed to the UK Animals (Scientific Procedures) Act (1986). We began by examining the effects of GABA on arterioles pre-constricted with Et-1 (10nM) in retinal whole mounts. Application of 100µM GABA evoked vasodilation in 39% of vessels, vasoconstriction in 8%, and no response in the remaining 53% of vessels. When the concentration of GABA was increased to 1mM, a larger percentage of vessels responded, with 59% displaying vasodilation, 18% vasoconstriction and 23% no response. 1mM GABA was found to have no effect on the diameter of isolated vessels pre constricted with 10nM Et-1. In whole-mounts, muscimol (10µM), a GABAA receptor agonist, caused vasodilation in 6% of vessels, vasoconstriction in 6% and no response in the remaining 88% of vessels. In contrast, arteriolar responses to GABAB receptor agonists more closely resembled those observed with 1mM GABA. Baclofen (100µM) elicited vasodilation in 50% of vessels, while the remaining 50% exhibited no response. Application of SKF97541 (100µM) induced vasodilation in 39% of vessels, vasoconstriction in 8%, and no response in the remaining 53%. The selective GABAC receptor agonist, 5-methyl-imidazole-4-acetate (100µM), failed to evoke a vasomotor response in any vessels tested. Following pre-incubation of the whole-mount preparations with the GABAA receptor inhibitor, bicuculline (100µM), 70% of vessels that had previously exhibited a vasodilator reaction to 1mM GABA displayed a second, similar-sized response. In the presence of the GABAB receptor antagonist, 2-hydroxysaclofen (100µM), all vessels that had previously been responsive to GABA failed to exhibit a vasodilator response. Bicuculline or 2-hydroxysaclofen was subsequently applied to retinal whole-mount preparations where the arterioles had not been pre-constricted with Et-1. Bicuculline caused vasoconstriction in 1 out 20 arterioles, whilst 2-hydroxysaclofen induced vasoconstriction in 4 out 31 arterioles. Our data demonstrate that GABA can exert a dual effect on arteriolar tone in the rat retina, inducing either vasoconstriction or vasodilation. These reactions are entirely dependent upon the presence of the adjacent retinal neuropile and appear largely dependent upon the activation of GABAB receptors. Despite these findings, we find little evidence to suggest that endogenous GABA makes a significant contribution to the regulation of basal vascular tone in the rat retina at least under the ex vivo conditions used in our experiments.