The purpose of this study was to investigate the role of K⁺ and Cl^– channels in the regulation of basal and endothelin-1 (Et-1)-evoked alterations in volumetric blood flow in the rat retinal circulation. Hooded Lister rats (male, 320-450g) were anaesthetised with an intraperitoneal injection of ketamine (130 mg kg^-1) and xylazine (17.5 mg kg^-1). Fifteen minutes prior to blood flow imaging, animals were intravitreally injected (10μl) with Hanks’ solution, the vasoconstrictor peptide Et-1, or ion channel inhibitors in the absence or presence of Et-1. A group of non injected controls was also included. Volumetric blood flow was assessed in arterioles within the superior regions of the retina using acridine orange leukocyte fluorography and fluorescein angiography by means of a confocal scanning laser ophthalmoscope (HRA2, Heidelberg Engineering, Germany). Data are presented as means ± SEM. The n values reported refer to the number of animals used for each experiment (14-32 arterioles per group). Statistical analyses were carried out using a nonparametric Kruskall-Wallis test and Dunn’s procedure for post hoc evaluation. In non-injected eyes, mean volumetric flow in single arterioles was 12.31 ± 0.23 nL s^-1 (n=9) and this was unchanged in eyes injected with Hanks’ solution (12.79 ± 0.25 nL s^-1, n=9, P>0.05). Injection of the Cl^– channel blocker, 4,4′- diisothiocyanatostilbene-2,2′-disulphonic acid (DIDS, 10 mM), increased volumetric flow (15.67±0.41 nL s^-1, n=9, P<0.001), while injection of penitrem A (1μM), an inhibitor of large-conductance Ca²⁺-activated K⁺ channels, or correolide (40μM), a Kv1 channel inhibitor, reduced flow rates (11.24±0.36 nL s^-1, n=11, P<0.01 and 11.31±0.28 nL s^-1; n=10, P<0.05, respectively). Intravitreal injection of Et-1 (104nM), induced severe vasoconstriction and a substantial reduction in flow (4.64±0.16 nL s^-1, n=8, p<0.001). This response was not affected by co-application of K⁺ channel blockers (p>0.05 vs Et-1 alone), but was reversed in the presence of DIDS (14.71±0.272 nL s^-1, n=7, p<0.001 vs Et-1). These results suggest that K⁺ and Cl^– channels are pivotal in controlling basal blood flow in the retina, while Cl^– channels may also play a key role in mediating the vasoregulatory actions of Et-1 in the retinal circulation.