The length of time that blood remains in the pulmonary capillary is an important variable in gas exchange. We have investigated the distribution of capillary transit times in isolated rabbit lungs perfused with a bicarbonate-free buffer. The time course of gas exchange was monitored by enclosing the lungs in a plethysmograph. A bolus of buffer containing dissolved acetylene was injected into the perfusion system. Exchange of this inert gas occurred as soon as the bolus reached the capillary bed, thereby describing the input function into the bed. A separate bolus injection of bicarbonate solution resulted in production and excretion of CO2 as long as the bolus remained in the capillary bed. The rate of CO2 production was adjusted by partial inhibition of endothelial carbonic anhydrase. The distribution of capillary transit times was computed from a model of CO2 production in the capillary bed and the observed rates and volumes of acetylene and carbon dioxide excretion. The recovered distributions indicate that there is a fairly wide distribution of capillary transit times (relative dispersion, 0.45) around the mean value of 1.71 s (+/- 0.53 [SD]). Only 10% of capillary transit times are less than one half of mean transit time. It is likely that gas exchange reaches equilibrium in the capillary bed except possibly during strenuous exercise or exposure to high altitude or in disease.