We have studied the effects of triiodothyronine administration (20-40 μg three times daily over one week) in six healthy young men, on the activities of lipoprotein lipase and hepatic lipase and on plasma lipoprotein concentrations. Hepatic lipase activity in post-heparin plasma rose by 46 ± 25% (p< 0.025), whereas the activity of lipoprotein lipase did not change significantly. Plasma cholesterol concentrations decreased by about 20% (p< 0.025), whereas there was no change in plasma triglyceride levels. The fall in plasma cholesterol could be accounted for by a reduction of HDL cholesterol (-11%, p< 0.025) as well as LDL cholesterol (-27%, p< 0.025). The data emphasize the role of hepatic lipase in the lipoprotein alterations associated with thyroid dysfunction.