The antagonism between prostaglandin and vasopressin represents a classic negative feedback loop. It is not clear whether cyclooxygenase (COX)-2 and/or COX-1 expression is involved in elevated prostaglandin production stimulated by vasopressin in vivo. In the present study, we explored vasopressin regulation of medullary COX-2 and COX-1 expression acutely and chronically in rats. Medullary COX-1 expression was moderately lower and COX-2 expression was significantly lower in adult male Brattleboro rats than age-matched Long-Evans controls. Chronic treatment of Brattleboro rats with vasopressin for 1 wk led to a decrease in urine volume and a moderate increase in medullary COX-1; in contrast, medullary COX-2 expression was almost undetectable in untreated rats but was dramatically up-regulated with vasopressin treatment and was accompanied by increased urinary prostaglandin E(2) excretion. Further investigation revealed that both V1 and V2 receptors were involved in chronic medullary COX-1 and COX-2 up-regulation. Acute treatment with specific V1 or V2 receptor agonists resulted in specific increases in medullary COX-2, which was prevented by furosemide. Vasopressin did not affect COX-2 expression in cultured renomedullary interstitial cells. These data demonstrate that vasopressin stimulates medullary COX-2 expression through activation of both V1 and V2 receptors, and this stimulation is indirect and probably involves increased medullary electrolyte tonicity.