Intoxication of rats with CCl4 (1 ml/kg) resulted in the almost complete loss of glutamine synthetase (GS) specific activity and immunologically detectable enzyme protein known to be expressed exclusively in some hepatocytes of the perivenous zone of the liver acinus. During regeneration the specific activity as well as the original number of GS-positive (GS+) hepatocytes were reestablished. However, while the GS+ hepatocytes in control livers were arranged in up to 3 cell layers surrounding the central veins the same number of GS+ hepatocytes in regenerated livers formed a single cell layer only, most likely because the central veins were enlarged in diameter. Investigation of the nuclear pattern of GS+ and GS- hepatocytes of control animals in primary cultures revealed striking differences characterized by significantly more mononuclear diploid, binuclear diploid, and binuclear tetraploid cells among the GS+ hepatocytes and predominantly mononuclear tetraploid cells (70%) among the GS- hepatocytes. Immediately after liver damage by CCl4 and during regeneration small but significant changes in the nuclear pattern were noted for GS- hepatocytes. However, the first GS+ cells appearing during early regeneration showed a pattern of ploidy classes close to the original one found for GS- hepatocytes. These results indicate that new GS+ hepatocytes may be derived from formerly GS- cells which are induced to express GS if they have reached the border of the central veins.