When rats were subjected to partial hepatectomy, glucosamine 6-phosphate synthase (EC 5.3.1.19) of the remaining liver underwent alterations both in activity and in molecular form. To study the molecular alterations, glucosamine 6-phosphate synthase was purified from regenerating as well as control liver and was analyzed by isoelectric focusing. Although control liver exhibited only one form of glucosamine 6-phosphate synthase with a pI of 5.0, sequential and transient appearance of three other forms, with pI values of 4.3, 4.8, and 4.5, respectively, was observed for regeneration liver within 72 hr following partial hepatectomy. Laparotomy, on the other hand, induced in the liver only the pI 4.8 form, and injection of a mixture containing triiodothyronine, amino acids, glucagon, and heparin induced only the pI 4.3 and 4.5 forms. It therefore appears that the pI 4.3 and 4.5 forms, but not the pI 4.8 form, are associated with hepatic DNA synthesis. The pI 4.8 form is induced in the liver in response to surgical stress.