gamma-Glutamyltranspeptidase is expressed at low levels in the liver of the male Fischer 344 rat where it exhibits 15-fold purification and 33% recovery in isolated plasma membranes. While the activity of the enzyme is unaltered in regenerating liver 24 h after partial hepatectomy, it increases steadily thereafter over a period of one week. Seven days after partial hepatectomy the enzyme is maximally activated: 5.6-fold in liver homogenates and 5.3-fold in isolated liver plasma membranes. The enzyme declines in activity over the next fourteen days and is expressed at normal levels three weeks after partial hepatectomy. These results demonstrate that the activity of gamma-glutamyltranspeptidase increases in regenerating liver but that the increase is out of phase with the proliferative response.