Hemodynamic, respiratory, and pathological changes that occur during experimental fat embolism with free fatty acid have been evaluated in mongrel dogs. Most important were the elevation of pulmonary artery and pulmonary artery wedge pressure combined with sustained falls in the pulmonary pressures distal to the pulmonary capillary bed, indicating obstruction at the capillary bed. Systemic blood pressure fell. Decreasing dynamic pulmonary compliance, hypoxemia, and metabolic acidosis have also been consistent. These changes reflect the severe damage to the pulmonary capillary bed secondary to the fatty acid embolus. Results of pathological examination of the lungs revealed marked interstitial and alveolar edema. As the experiment progressed severe erythrocytic clumping and almost complete cessation of pulmonary capillary blood flow became obvious. The proposal that fatty acids are the underlying cause of the findings in tat embolization has been examined.