gamma-Glutamyltranspeptidase displays the following order of activity in tissues of the Fischer 344 rat: kidney much greater than small intestine much greater than cerebral cortex = testis greater than lung much greater than liver = heart. The activity of the hepatic enzyme in rats is: 4-fold higher in females than males; 4-fold higher in male Wistar, Sprague-Dawley and Zucker rats than male Fischer 344 rats; increased 10-fold in very old vs young male Fischer 344. The hepatic enzyme displays significant species variation: the mouse and rat liver enzymes are similar and low in activity, while duck, dog, pig and beef enzymes are 7, 13, 86 and 92-fold higher, respectively, in activity than the male Fischer rat liver enzyme. A liver plasma membrane isolation procedure has been devised which selects for the sinusoidal face of the liver parenchymal cell as assessed by marker enzyme analysis: for these plasma membranes the purification of gamma-glutamyltranspeptidase is 21.5 and the recovery is 42% indicating that this is the cellular and subcellular locus of the enzyme in rat liver. The characteristics of the liver plasma membrane from female rats are: pH optimum of 8.0; classical Michaelis-Menten kinetics; Km of 1.43 mM and Vmax of 33.3 nmol X mg-1 X min-1. In Fischer 344 rats, gamma-glutamyltranspeptidase activities are elevated over adult levels in perinatal liver: in fetal liver homogenates and plasma membranes the activities are increased 179 and 109-fold, respectively. The activity peaks just after birth and declines rapidly over the first 15 postnatal days. The activity of the liver enzyme in the male Fischer 344 rat exhibits a progressive increase throughout diethylnitrosamine-induced hepatocarcinogenesis: it is increased 7.8-fold in homogenates and 5.4-fold in plasma membranes at the early premalignant stage; 74-fold in homogenates and 31-fold in plasma membranes at the later hyperplastic nodular premalignant stage; and 174-fold in homogenates and 61-fold in plasma membranes at the hepatoma stage. The gradual drop in purification during hepatocarcinogenesis is associated with the appearance of the enzyme in the blood.