Collecting duct (CD)-derived endothelin-1 (ET-1) exerts natriuretic, diuretic and hypotensive effects. In vitro studies have implicated cyclooxygenase metabolites, and particularly PGE2, as important mediators of CD ET-1 effects. However, it is unknown whether PGE2 mediates CD-derived ET-1 actions in vivo. To test this, CD ET-1 knockout (KO) and control mice were studied. During normal salt and water intake, urinary PGE2 excretion was unexpectedly increased in CD ET-1 KO mice. Salt loading markedly increased urinary PGE2 excretion in both groups of mice, however the levels remained relatively higher in knockout animals. Acutely isolated inner medullary CD (IMCD) from knockout mice also had increased PGE2 production. The increased IMCD PGE2 was COX2-dependent since NS398 blocked all PGE2 production. However, increased CD ET-1 KO COX2 protein or mRNA could not be detected in inner medulla, or IMCD, respectively. Inner medullary COX1 mRNA and protein, and IMCD COX1 mRNA, levels were unaffected by Na intake or CD ET-1 KO. Knockout mice on a normal or high Na diet had elevated blood pressure; this difference was not altered by indomethacin or NS398 treatment. However, indomethacin or NS398 did increase urine osmolality and reduce urine volume in knockout, but not control, animals. In summary, IMCD COX2-dependent PGE2 production is increased in CD ET-1 KO, indicating that CD-derived ET-1 is not a primary regulator of IMCD PGE2. Further, the increased PGE2 in CD ET-1 KO mice partly compensates for loss of ET-1 with respect to maintaining urinary water excretion, but not in blood pressure control.