Abstract-Prostaglandins participate in the regulation of sodium and water renal excretion. They are synthesized by cyclooxygenases (COX): the constitutive isoform and the enzyme regulated by physiological stimuli (COX-2). Our previous immunohistochemical studies have demonstrated the presence of COX-2 in a subset of thick ascending limb (TAL) of Henle cells and its induction during the postnatal period and after adrenalectomy. Previous results suggested that this induction phenomenon proceeds by recruitment of TAL cells from the cortex to the outer medulla. The present work aimed to specifically address these preliminary observations by using immunohistochemical techniques in single microdissected nephron segments. Normal adult rats, adrenalectomized rats, adrenalectomized rats on dexamethasone and 5, 10, and 15 days postnatal age were used (Sprague-Dawley rats, nϭ 5 each group). Glomeruli and different segments of nephron were microdissected from collagenase-treated kidney tissue. Tubules were immunostained with specific antibodies against COX-2. We confirmed that COX-2 was localized exclusively in TAL segments; it was induced after adrenalectomy and during postnatal age, peaking at 15 days after birth. We provided morphological evidence that the induction of COX-2 along TAL proceeded in a defined pattern by recruitment of cells from the cortical portion close to the glomeruli toward the outer medulla. No COX-2 was observed in the post-macula densa portion of the segments. Our results provide the anatomical basis for the contribution of COX-2 in physiological mechanisms such as renin secretion, tubuloglomerular feedback, and the interaction with neuronal NO synthase at the juxtaglomerular apparatus. Key Words: cyclooxygenase Ⅲ kidney Ⅲ rat Ⅲ immuno histochemistry Ⅲ glucocorticoids Ⅲ prostaglandins I n the kidney, eicosanoids act as paracrine agents that modulate kinins actions and vasoactive hormones and regulate several aspects of renal function: renal blood flow and hemodynamics 1 and transepithelial NaCl transport in several nephron segments. 2 Prostaglandin production depends on activation of the PGG 2 /H 2 synthase, also known as cyclooxygenase (COX). Two distinct isoforms of this enzyme have been described: the constitutive isoform (COX-1) and the regulated isoform (COX-2). In the kidney, COX-1 is found in arterial vascular endothelial cells, medullary and cortical collecting ducts, and medullary interstitial cells. 3 The COX-2 isoform is commonly known as an inductive enzyme that expresses in conditions such as inflammation and tissue damage. 4 Previously, our laboratory demonstrated the presence of COX-2 in thick ascending limb (TAL) of Henle cells in normal adult rat, 5 its regulation during the postnatal development, 6 and its regulation by glucocorticoid. 7 There is also evidence that COX-2 is regulated by dietary salt intake, 8,9 angiotensin II, 10 corticosteroid, 11 and furosemide. 12 Furthermore, COX-2-derived prostaglandins participate in renin secretion, 13,14 tubuloglomerular feedback, 15 and s...