R ecent evidence has implicated endothelin-1 (ET-1) as an autocrine inhibitor of inner medullary collecting duct (IMCD) sodium and water transport. The regulators of IMCD ET-1 production are, however, largely unknown. Because of the unique hypertonic environment of the IMCD, the effect of varying extracellular tonicity on IMCD ET-1 production was evaluated. Increasing media osmolality from 300 to 450 mosmol with NaCl or mannitol but not urea caused a marked dose- and time-dependent reduction in ET-1 release by and ET-1 mRNA in cultured rat IMCD cells. In contrast, increasing osmolality had no effect on ET-1 production by rat endothelial or mesangial cells. To see if ET-1 varies in a similar manner in vivo, ET-1 production was assessed in volume expanded (lower medullary tonicity) or volume depleted (high medullary tonicity) rats. Urinary ET-1 excretion and inner medulla ET-1 mRNA were significantly reduced in volume depleted as compared to volume expanded animals. These results indicate that extracellular sodium concentration inhibits ET-1 production specifically in IMCD cells. We speculate that extracellular sodium concentration, via regulation of ET-1 production, provides a link between volume status and IMCD sodium and water reabsorption.