Investigations in guinea pigs to evaluate interferences between cholestasis and enzymatic induction by phenobarbital, and influences that both experimental conditions exert on the levels of gamma-glutamyl transpeptidase in microsomes and other subcellular fractions, have shown that: 1) cholestasis inhibits inductions of cytochrome P-450 from phenobarbital; 2) gamma-glutamyl transpeptidase is not an inducible enzyme in guinea pig; 3) rises or ferum gamma-glutanyl transpeptidase produced either by cholestasis or phenobarbital administration, are not related to an increased enzymatic synthesis in the various subcellular fractions of liver cell.