Recently, we reported near-complete repopulation of the rat liver by transplanted hepatocytes using retrorsine (RS), a pyrrolizidine alkaloid that alkylates cellular DNA and blocks proliferation of resident hepatocytes, followed by transplantation of normal hepatocytes in conjunction with two- thirds partial hepatectomy (PH). Because two-thirds PH is not feasible for use in humans, in the present study, we evaluated the ability of thyroid hormone (triiodothyronine [T3]), a known hepatic mitogen, to stimulate liver repopulation in the retrorsine model. Because T3 initiates morphogenesis in amphibians through a process involving both cell proliferation and apoptosis, we also determined whether apoptosis might play a role in the mechanism of hepatocyte proliferation induced by T3. Following hepatocyte transplantation and repeated injections of T3, the number of transplanted hepatocytes in the liver of RS-pretreated animals increased progressively to repopulate 60% to 80% of parenchymal cell mass in 60 days. We show further that T3 treatment augments proliferation of normal hepatocytes, as evidenced by increased histone 3 mRNA and cyclin-dependent kinase 2 (cdk2) expression, and this is followed by apoptosis. These combined effects of T3 lead to selective proliferation of transplanted hepatocytes in RS-pretreated rats, while endogenous hepatocytes, which are blocked in their proliferative capacity by RS, mainly undergo apoptosis. Thus, T3 can replace PH in the RS-based rat liver repopulation model and therefore represents a significant advance in developing methods for hepatocyte transplantation.