The role of endothelium-derived hyperpolarizing factor (EDHF) in the regulation of blood flow in vivo was examined in the mesenteric and hindlimb circulations of anaesthetized rats. Basal mesenteric conductance decreased from 57 ± 5 to 20 ± 6 μl min-1 mmHg-1 when nitric oxide (NO) production was inhibited, and combined blockade of intermediate- and small-conductance Ca2+t́ivated K+ (KCa) channels with charybdotoxin (ChTx) and apamin had no further effect. Basal hindlimb conductance was reduced from 39 ± 3 to 22 ± 2 μl min-1 mmHg-1 by NO synthesis inhibition, with no effect of the KCa channel blockers. Endothelial stimulation with acetylcholine (ACh) infusion directly into the mesenteric bed increased conductance by 20 ± 2 μl min-1 mmHg-1. Blockade of NO synthesis decreased this conductance to 15 ± 1 μl min-1 mmHg-1 leaving the response attributable to EDHF. This was reduced to 2 ± 1 μl min-1 mmHg-1 by ChTx plus apamin but not by iberiotoxin, which selectively blocks large-conductance KCa channels. Similar results were obtained when bradykinin (BK) was used to stimulate the endothelium. Nitroprusside, which directly relaxes smooth muscle, evoked an increase in conductance that was resistant to all blockers tested. ACh-induced increases in hindlimb conductance were reduced from 19 ± 1 to 12 ± 1 μl min-1 mmHg-1 by NO synthesis inhibition and further reduced to 2 ± 2 μl min-1 mmHg-1 by ChTx plus apamin. In contrast to NO, ChTx- and apamin-sensitive EDHF appears to contribute little to basal conductance in rat mesenteric and hindlimb beds. However, EDHF accounts for a significant component of the conductance increase during endothelial stimulation by ACh and BK. In these beds, intermediate- and small-conductance KCa channels underpin EDHF-mediated vasodilatation.