1. Acetone-dried powders of liver and heart tissues from rats given a high-carbohydrate diet or a fat meal were assayed for lipoprotein lipase activity. Heart tissue showed typical lipoprotein lipase activity, whereas none was detected in liver by the usual assay procedures. 2. When mixed acetone-dried powders were prepared from heart plus liver, there was a marked suppression of the expected activity, indicating that an inhibitor was present in the liver. This inhibition was partially overcome in the presence of relatively large amounts of heparin. 3. Lipoprotein lipase was also detected in liver alone when large quantities of heparin were added to the assay system. 4. No increase in lipoprotein lipase activity in either liver or heart was detected when rats were given a fat meal. 5. It is concluded that the liver of the rat contains lipoprotein lipase that is normally present in an inactive state. The results imply that a heparinase is the agent responsible for the inactivation. 6. The significance of the non-functional status of lipoprotein lipase in the liver is discussed. The results support the view that direct hydrolysis of plasma triglycerides by the liver is not a significant physiological process.

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