The analysis of the gas in a single expirate has long been used to estimate of the degree of ventilation-perfusion inequality in the lung. To further validate this, we examined three measures of V_A/Q inhomogeneity calculated from a single full exhalation in 9 anesthetized mongrel dogs under control conditions and after exposure to aerosolized methacholine. These measures were then compared to arterial blood gases, and to measurements of V_A/Q inhomogeneity obtained using the multiple inert gas elimination technique. The slope of the instantaneous respiratory exchange ratio (R-slope) against expired volume was poorly correlated with independent measures, probably because of the curvilinear nature of the relationship due to continuing gas exchange. When R was converted to the intra-breath V_A/Q (iV/Q), the best index was the slope of iV/Q against volume over phase III (iV/Q-slope). This was strongly correlated with independent measures, especially those relating to inhomogeneity of perfusion. The correlations for both iV/Q-slope and R-slope considerably improved when only the first half of phase III was considered. We conclude that a useful non-invasive measurement of V_A/Q inhomogeneity can be derived from the intra-breath respiratory exchange ratio.