Renal oxygen consumption ((V) over dot O-2,O-ren) is an important parameter that has been shown to be influenced by various pathophysiological circumstances. (V) over dot O-2,O-ren has to be repeatedly measured during an experiment to gain insight in the dynamics of (dys) regulation of oxygen metabolism. In small animals, the classical approach of blood gas analysis of arterial and venous blood samples is only limitedly applicable due to fragile vessels and a low circulating blood volume. We present a phosphorescence lifetime technique that allows near-continuous measurement of renal venous PO2 (vPO(2)) and (V) over dot O-2,(ren) in rats. The technique does not rely on penetration of the blood vessel, but uses a small reflection probe. This probe is placed in close proximity to the renal vein for detection of the oxygen- dependent phosphorescence of the injected water- soluble near- infrared phosphor Oxyphor G2. The technique was calibrated in vitro and the calibration constants were validated in vivo in anesthetized and mechanically ventilated male Wistar rats. The hemoglobin saturation curve and its pH dependency were determined for calculation of renal venous oxygen content. The phosphorescence technique was in good agreement with blood gas analysis of renal venous blood samples, for both PO2 and hemoglobin saturation. To demonstrate its feasibility in practice, the technique was used in four rats during endotoxin infusion (10 mg center dot kg(-1)center dot h(-1) during 1 h). Renal vPO(2) reduced by 40% upon reduction in oxygen delivery to 30% of baseline, but (V) over dot O-2 remained unchanged. This study documents the feasibility of near- continuous, nondestructive measurement of renal vPO(2) and (V) over dot O-2 by oxygen-dependent quenching of phosphorescence.