Body weight gain, food intake and serum concentrations of total lipids, cholesterol, triglycerides, phospholipids and free fatty acids were measured in genetically obese and lean female Zucker rats after surgical thyroidectomy and treatment with 3,5,3'-triiodothyronine (T3) (3 micrograms/100 g body wt per day) for 4 wk. There were large differences in the major serum lipids between the control obese and lean rats. Obese rats were characterized by high levels of the various categories of lipids, especially of triglycerides. Thyroidectomized rats had higher serum concentrations of total lipids and cholesterol than controls in both obese and lean rats. The serum concentration of triglycerides was higher in the thyroidectomized lean rats than in lean controls but lower in their thyroidectomized obese littermates than in obese controls. The T3 treatment resulted in a marked reduction in serum total lipid, cholesterol, triglyceride and phospholipid levels in both thyroidectomized and control obese rats. The same treatment given to thyroidectomized and control lean rats also produced a lower concentration in serum total lipid, cholesterol and triglyceride levels but had no significant effect on serum phospholipid levels. The serum concentration of free fatty acid was not significantly affected by T3 administration in either obese or lean rats. Thyroidectomy induced a lower body weight gain in both obese and lean rats. The T3 treatment restored the body weight gain in the thyroidectomized lean rats but not in the thyroidectomized obese rats, although the food intake was increased by T3.(ABSTRACT TRUNCATED AT 250 WORDS)