Endothelin-1 (ET-1) is increased in rats on a high salt (HS) diet and participates in salt-dependent hypertension. Afferent arterioles (AA) are important for long-term blood pressure control, and therefore we hypothesized that a HS diet would alter their responsiveness to ET-1. Sprague-Dawley rats were fed either normal salt (NS, 0.66% NaCl) or HS (8%) diet for one week. Diameters of AA were determined in response to increasing concentrations of big ET-1, ET-1, sarafatoxin (S6c) or norepinephrine (NE), using the blood perfused juxtamedullary nephron technique. ET-1 responses were also determined during blockade of endothelin type A (ET_A) or type B (ET_B) receptors with 10nM ABT-627 or 30nM A-192621, respectively. Expression of ET_A and ET_B receptors was determined in renal microvessels. Responses of AA to big ET-1, ET-1 and S6c were significantly attenuated during a HS diet (e.g. response to 10^-10 M ET-1 in NS versus HS rats: -52.5±10.2% versus +5.6±11.3% of control diameter; p0.05), with no change in the responses to NE. ET_B, but not ET_A receptor blockade abolished the different response to ET-1 between NS and HS groups. ET_B receptor expression in renal microvessels was increased in response to HS (17.7±2.4 vs. 6.6±3.0% of -actin, p=0.02), whereas ET_A receptor expression was unchanged. These results suggest that the reduced vasoconstrictor response of AA to endothelin peptides during a HS diet is mediated by increased vasodilatory function of endothelial ET_B receptors. By preserving renal blood flow, this may be an important mechanism to restore sodium balance during a HS diet.