The effect of glucagon on serum lipids and very low density lipoproteins (VLDL) has been examined in the eulipemic and the hyperlipemic rat. An inhibition of amino acid incorporation into hepatic lipoprotein apoprotein was observed, with an associated decrease in circulating VLDL apoprotein, decreased serum triglyceride concentration, and a loss of the pre-beta band as judged by serum lipoprotein electrophoresis. The data suggest that an important action of this hormone is to decrease the synthesis of the protein moiety of the VLDL; this may contribute to the hypolipemic action of glucagon by introducing a limitation in hepatic lipoprotein production.