This study compared the renal metabolism of arachidonic acid in Brattleboro (BB) (vasopressin-deficient) and Long-Evans (LE) control rats and the effects of a cytochrome P450 (CYP) inhibitor, 1-aminobenzotriazole (ABT) on renal function in these animals. The production of 20-hydroxyeicosatetraenoic acid (20-HETE) by renal cortical and outer medullary microsomes was significantly greater in BB than in LE rats (155±16 vs. 92±13 and 59±7 vs. 33±3 pmol/min/mg protein). Renal cortical epoxygenase activity was not different in these strains. The expression of CYP4A proteins was 58% and 78% higher in the renal cortex and outer medulla of BB than in LE rats. Chronic treatment of BB rats with a vasopressin type 2 receptor agonist for one week normalized the renal production of 20-HETE. Chronic blockade of the formation of 20-HETE and EETs with ABT had little effects on renal function in LE rats. However, urine flow increased by 54% and urine osmolarity decreased by 33% in BB rats treated with ABT. Plasma levels of oxytocin fell significantly from 7.2±1.3 to 3.9±1.0 pg/ml. The effects of ABT in BB rats were attenuated by chronic infusing of oxytocin (0.7ng/min/100g) to maintain fixed high plasma levels of this hormone. These results indicate that the expression of CYP4A protein and the renal formation of 20-HETE are elevated in the kidney of BB rats due to a lack of vasopressin, and that chronic blockade of the formation of 20-HETE and EETs with ABT promotes water excretion in vasopressin-deficient BB rats by reducing the circulating levels of oxytocin which is a weak vasopressin agonist.