The effect of insulin on cell proliferation in vivo has been studied in hepatectomised streptozotocin-diabetic rats. The extent of cell proliferation in sham and hepatectomized-control, diabetic and insulin treated rats were monitored by determining DNA content and [3H]thymidine incorporation into DNA. The kinetic parameters of thymidine kinase a regulatory enzyme for DNA synthesis was also studied in these groups. The rate of DNA synthesis in liver of streptozotocin-diabetic rats was significantly higher 24 hrs post-hepatectomy compared to control and insulin treated diabetic groups. Kinetic studies of thymidine kinase revealed that there was no change in the Michaelis-Menten constant (Km) whereas maximum velocity (Vmax) was elevated in the diabetic hepatectomized groups compared to control and insulin treated hepatectomized groups. Thus our study elucidates the role of insulin in thymidine kinase activity and DNA synthesis.