SUMMARY Blood sugar, the serum concentration of total lipids, total glycerol, cholesterol and phosphatides were determined in chickens from the Obese strain (OS) of White Leghorns. Over 90% of these chickens are afflicted with a spontaneously occurring autoimmune thyroiditis and show clinical symptoms of hypothyroidism. Blood glucose levels were increased above normal limits in OS chickens up to 5 weeks of age. Significantly increased serum lipid concentrations were found in the OS chickens at the ages of 1, 5, 8 and 15 weeks, while 3-week-old OS chickens showed almost normal serum lipid levels. A single intravenous injection of heparin in a dose which prolonged bleeding time, had no 'clearing effect' on the serum of OS birds. Insulin given subcutaneously for 4 days had no effect on blood sugar but decreased the serum phosphatides and total lipid concentrations. The short-term administration of high doses of thyroxine significantly lowered the serum levels of total lipids, cholesterol, phosphatides and total glycerol.