The effect of positive end-expiratory pressure (PEEP) on plasma renin activity (PRA), renal function, and cardiovascular (CV) hemodynamics during high frequency jet ventilation (HFJV) was observed in 7 patients. The addition of PEEP during HFJV increased PRA while decreasing stroke index (SI) and cardiac index (CI). These changes were associated with decreased urinary flow, creatinine clearance, and fractional excretion of sodium. In contrast, HFJV at zero end-expiratory pressure (ZEEP) maintained normal PRA, renal function, and CV hemodynamics. The authors conclude that the alteration of renal function during HFJV is a function of airway pressure rather than the effects of the ventilatory frequency. The deterioration of renal function may have been due to changes in PRA or CV dynamics.