Gas exchange following moderately severe experimental pulmonary fat embolism was studied in nine dogs. A new method designed to describe the distribution of ventilation-perfusion ratios in the lung was applied before and after intravenous injection of homologous neutral fat. The dose of fat (0.75 ml/kg) was low enough to produce a small but significant decrease in arterial PO2 (mean change of 10 mm Hg) in the first 15 minutes after the embolism but high enough to result in the death of two of the four dogs that were allowed to survive the initial postembolism period. Pulmonary artery pressure and pulmonary vascular resistance both rose significantly within 5 minutes of the fat injection and remained elevated for the 2 hour experimental period. Immediately after the embolism there was an increase in the percentage of the total ventilation going to areas of the lung with ventilation-perfusion ratios between 10 and 100, which usually appeared as a discrete mode in the ventilation distribution. This mismatching of ventilation and perfusion partially resolved within 2 hours after the embolism, as indicated by the gradual disappearance of this population of gas exchanging units with relatively decreased blood flow. At no time within 2 hours after the embolism was there a significant increase in shunt or in ventilation to totally unperfused lung. The gas exchange pattern in the two dogs that subsequently died was indistinguishable from that of the other seven in the immediate postembolism period.