Lipid and lipoprotein concentration, and triglyceride turnover were studied in control, thyroidectomized and pair-fed control rats (pair-fed to match the food intake of the thyroidectomized rats). Thyroidectomy induced a significant increase in plasma cholesterol (and low density lipoprotein) concentrations and a decrease in plasma triglyceride (and very low density lipoprotein) concentrations. Changes in similar direction, but of smaller magnitude were observed in the plasma of the pair-fed control rats. To further investigate triglyceride metabolism in these 3 groups of animals, triglyceride turnover was studied in fasted, unrestrained and unanesthetized rats, following injection of [2-3H]glycerol. Peak incorporation of [2-3H]glycerol into plasma triglyceride occurred in all 3 groups of animals at 25 min after precursor administration, although the maximal incorporation was substantially lower in the thyroidectomized group than in either of the control groups. Thereafter, plasma triglyceride radioactivity decayed monoexponentially with a half-life of 41 .+-. 3 min observed in the thyroidectomized rats. The calculated apparent fractional catabolic rates were thus 0.029 min-1 for both control groups and only 0.017 min-1 for the thyroidectomized animals. The apparent total catabolic rates of plasma triglyceride were 299 .+-. 11, 138 .+-. 11 and 48 .+-. 4 .mu.g triglyceride .cntdot. min-1 for the normal controls, pair-fed controls and thyroidectomized rats, respectively. The importance of thyroid hormones in regulating plasma lipid and lipoprotein metabolism is emphasized; hypothyroidism results in a reduction of triglyceride secretion into, and the removal from, circulation. The decreased caloric intake of the hypothyroid animals evidently cannot, in itself, account for this observation.