The renal clearances (C) of p-aminohippurate (PAH) and tetraethylammonium (TEA) as markers for renal plasma flow (RPF) were compared in rats under different experimental conditions. CTEA and CPAH were of the same magnitude and varied synchronously during volume expansion and anesthesia. The renal extractions of the two substances were closely correlated and averaged 85.9% for PAH and 88.7% for TEA (p less than 0.01). High doses of furosemide (120 mg/kg as intravenous bolus) reversibly decreased the CPAH/CTEA ratio, suggesting that furosemide may compete for PAH secretion in the proximal tubule. A similar effect was observed after administration of probenecid. The results indicate that TEA is a more reliable indicator of RPF than PAH in studies involving accumulation of endogenous or exogenous organic anions.