In the adult mammalian kidney, high levels of cyclooxygenase (COX)-2 expression can be detected in the macula densa and associated cortical thick ascending limb cells and medullary interstitial cells. In the renal cortex, COX-2 expression increases in high renin states, and selective COX-2 inhibitors significantly decrease plasma renin levels. In the medullary region of the kidney, the expression of COX-2 increases in response to a high-salt diet and water deprivation. The most important prostanoids in the kidney are prostaglandin (PG)I₂, or prostacyclin, and PGE₂. PGE₂ diminishes sodium reabsorption; thereby, its inhibition results in sodium retention that can manifest clinically in a variety of ways, such as peripheral edema, increased blood pressure (mainly in treated hypertensive patients), weight gain, and occasionally deterioration of heart failure. PGI₂ increases potassium secretion. As such, its inhibition can result in hyperkalemia, particularly in patients with underlying renal insufficiency. PGI₂ is also a potent vasodilator and helps maintain renal perfusion in conditions of decreased actual or effective circulating volume; its inhibition in such patients can result in acute renal failure. A variety of studies has been conducted to examine the effects of celecoxib and rofecoxib on renal function. These incorporate various study designs directly, making it virtually impossible to compare data across studies. It is apparent from such studies, coupled with published case reports, that the impact of both celecoxib and rofecoxib on renal function (including development of edema and hypertension) is similar to that of nonselective nonsteroidal anti-inflammatory drugs (NSAIDs). Studies comparing the 2 COX-2 inhibitors conflict in their interpretation. Overall, the data suggest similar effects on renal function among all NSAIDs when used at comparable doses.