The role of renal prostaglandins (PGE2 and PGF2 alpha) and kallikrein in the renal escape from excess mineralocorticoid has been evaluated in 10 normal volunteers on a high sodium diet. Escape from 9-alpha-fluorohydrocortisone (9-alpha-FF) administered for 12 days (0.6 mg daily) occurred within 4-5 days when 345 +/- 50 mmol sodium had accumulated. Indomethacin, a cyclooxygenase inhibitor, was then administered for 3 days while maintaining 9-alpha-FF. This was followed by a further cumulative gain of 105 +/- 28 mmol sodium after which escape resumed. Potassium balance remained negative throughout the study. Both PGE2 and PGF2 alpha excretion increased significantly under 9-alpha-FF from respective control values of 220 +/- 50 and 818 +/- 86 ng/24 h. to 610 +/- 317 and 1213 +/- 132 ng/24 h at the time of escape. Concomitantly urinary kallikrein increased from 1.1 +/- 0.2 to 2.5 +/- 0.3 units/24 h. Creatinine clearance increased from a mean baseline value of 111 +/- 4 to 123 +/- 4 ml/min during the same period. Indomethacin produced an inhibition of prostaglandin excretion without significant alteration of kallikrein excretion. Overall these results and particularly the transient sodium retention induced by indomethacin at the time of mineralocorticoid escape, suggest that prostaglandins may contribute to the escape phenomenon, whereas the role of kallikrein does not appear to be determinant.