The role of an endothelium-derived hyperpolarizing factor (EDHF), acting through the opening of vascular calcium-activated potassium (K(Ca)) channels, in the regulation of the basal diameter of human peripheral conduit arteries has never been investigated in vivo. We measured in 7 healthy subjects the effect of the local infusion of an inhibitor of K(Ca) channels, tetraethylammonium chloride (TEA, 9 micromol/min, 8 min), on radial artery diameter (echotracking) and flow (Doppler). Endothelium-independent dilatation was assessed before and after TEA using sodium nitroprusside (SNP: 5, 10 and 15 nmol/min, 3 min each). TEA induced a decrease in radial artery diameter (2.65 +/- 0.09 to 2.52 +/- 0.09 mm: p < 0.05) and flow (9.4 +/- 1.2 to 7.4 +/- 1.1 ml/min; p < 0.01) without modification in the radial artery dilatation in response to SNP (NS). The decrease in radial artery diameter was still significant even when the decrease in flow was taken as covariate into analysis (p < 0.05). These results demonstrate the role of vascular K(Ca) channels in the regulation of basal peripheral conduit artery diameter and arteriolar tone in human strongly suggesting the involvement of an EDHF a these two levels.