The liver plays a pivotal role in the handling of a wide variety of naturally occurring and xenobiotic or unnatural compounds. Some compounds such as proteins, triglycerides, phospholipids, glucose, and cholesterol are partly or wholly synthesized in the liver and supplied to the blood. Other relatively nonpolar compounds are converted to metabolites that are conjugated to sugars or amino acids. The resulting derivatives of increased polarity are then more efficiently excreted by the kidneys or the liver. In addition to these synthetic and metabolic activities, the liver also acts as an excretory organ. In this capacity, naturally occurring compounds such as bile acids and bilirubin are removed from the blood and excreted into bile. Similar handling of xenobiotics such as easily measured phthalein and fluorescein dyes has provided a classical means for nonradioactive evaluation of hepatocyte function.