We examined how the presence of a fixed level of basal renal oxygen consumption (VO₂^basal; oxygen used for processes independent of sodium transport) confounds the utility of the ratio of sodium reabsorption (T_Na+) to total renal VO₂ (VO₂^basal) as an index of the efficiency of oxygen utilization for sodium reabsorption. We performed a systematic review and additional experiments in anesthetized rabbits to obtain the best possible estimate of the fractional contribution of VO₂^basal to VO₂^basal; under physiological conditions (basal % renal VO₂). Estimates of basal % renal VO₂ from 24 studies varied from 0% to 81.5%. Basal % renal VO2 varied with the fractional excretion of sodium (FE_Na+) in the 14 studies in which FE_Na+ was measured under control conditions. Linear regression analysis predicted a basal % renal VO₂ of 12.7% to 16.5% when FE_Na+ = 1% (r2 = 0.48, P = 0.001). Experimentally induced changes in TNa+ altered T_Na+/VO₂^total in a manner consistent with theoretical predictions. We conclude that, because VO₂^basal represents a significant proportion of VO₂^total, T_Na+/VO₂^total can change markedly when T_Na+ itself changes. Therefore, caution should be shown when T_Na+/VO₂^total is interpreted as a measure of the efficiency of oxygen utilization for sodium reabsorption, particularly under experimental conditions where T_Na+ or VO₂^total changes.