The effect of insulin on the triglyceride deposits found in the pulmonary artery branches of streptozotocin-diabetic rats was investigated by treating the animals for two, five, nine or 14 days with insulin (3--8 units/day). Histochemical analysis showed that the triglyceride deposits in the pulmonary artery developed within three to four days after the induction of diabetes, but were not present in any animals five days from the initiation of insulin therapy. Plasma triglycerides, non-esterified fatty acids, phospholipid and total cholesterol concentrations were within the normal range within two days of the inception of insulin therapy and random plasma glucose levels were normal within five days. Analysis of lung lipids showed that after 14 days of insulin treatment the decreased content of phospholipids and the increased content of non-esterified fatty acids found in diabetic rats were also normalized. These findings suggest that insulin has an important role in the regulation of lipid metabolism in the pulmonary artery and lung tissue in the diabetic state.