Measurement of gamma-glutamyltransferase (GGT) activity in plasma is widely used in clinical biology (in order to detect hepatic diseases or to monitor treatment for alcoholism), and also in pharmacology (since this test is the only plasmatic marker for hepatic induction in human). However, the correct interpretation of a high plasmatic activity should take into account the various analytical factors which can affect results, as well as the physiological parameters known to modify this activity. It also requires its comparison to defined reference values. Several mechanisms may be involved in the increase of plasmatic activity as an index of hepatic induction, such as an increase in the protein synthesis, a release of the enzyme from the membrane or a modification in the biliary flux.