We have produced a line of transgenic mice expressing human hepatic triglyceride lipase (hH-TGL) to examine the in vivo effects of hepatic lipase expression on high density lipoprotein catabolism. Activation of metallothionine I promoter-hH-TGL cDNA transgene produced high levels of lipase mRNA in liver, heart, and kidney and elevated enzyme activity as assayed in post-heparin plasma. In a series of hyperlipidemic diet studies in which zinc was included in the diet to induce the transgene, hH-TGL expression was associated with a 34% lowering of plasma HDL-cholesterol levels (p < 0.01) when compared with animals on the same hyperlipidemic diet without zinc. This lowering of HDL cholesterol was paralleled by a decrease in total cholesterol and a decrease in HDL particle size. SDS-polyacrylamide gel electrophoresis analysis of the smaller HDL particles revealed that apolipoprotein AI was still the major apoprotein associated with the HDL. Quantitative analysis of abdominal aortic cholesterol content from the same animals suggests that the observed changes in plasma HDL by hH-TGL over-expression correlated with a decrease in the accumulation of aortic cholesterol (42%, p < 0.01). These data support the hypothesis that hH-TGL mediates a non-receptor pathway for the clearance of cholesterol from the plasma compartment.