To investigate the effect of diabetes on the lipid composition of the lungs and of the pulmonary artery, 43 streptozotocin diabetic rats and 43 control rats were examined. Triglyceride deposits were observed by a histochemical method in the branches of the pulmonary artery in 10 diabetic rats but in none of the controls. In the pulmonary tissue of the diabetic rats the total lipid content was not different from that of control animals, but the relative amount of phospholipids was decreased (p less than 0.001), and that of non-esterified fatty acids (p less than 0.001) and triglycerides (p less than 0.05) increased as compared to the control rats. These results indicate abnormalities in the lipid metabolism of the pulmonary artery and lungs during insulin deficiency.