Adrenal steroids are crucial for the progressive development of urinary concentrating ability in rats during the second to fourth postnatal weeks. Adrenal steroids suppress renal cyclooxygenase-2 (COX-2) expression in the postnatal period. In adult rat kidney prostaglandins lower concentrating ability by suppressing aquaporins and loop of Henle-associated sodium transporters. We hypothesized that removal of the adrenal glands in the postnatal period inhibits the development of urinary concentrating ability by increased COX-2 activity. At postnatal day 10 (P10) rat pups were adrenalectomized (ADX) or sham operated. ADX rats received daily injections of isotonic NaCl to sustain survival. From P17 to P20 (weaning) a group of ADX rats was given injections with the COX-2 selective blocker parecoxib (5 mg/kg*day). Rat pups were sacrificed at P20 after 22 h of water deprivation. ADX reduced body growth rate and significantly increased plasma renin concentration. Renal COX-2 mRNA and protein abundance was enhanced by ADX and immunohistochemical labeling revealed distribution along the full length of cTAL. Urinary concentrating capacity was reduced in ADX animals measured as a lower medullary interstitial osmolality and a lower urinary osmolality. Treatment of ADX animals with parecoxib resulted in a significantly increased papillary osmolality and increased urinary osmolality. In addition parecoxib-treatment improved body weight gain and significantly lowered plasma renin concentration indicating an improved ability to maintain extracellular volume. We conclude that lack of corticosteroid signaling in the postnatal period before weaning leads to impaired urinary concentrating ability and that enhanced renal COX-2 activity contributes to the salt-loosing phenotype.