This study was performed for assessing carcinogenicity of several synthetic hormones; Diethylstilbesterol (DES), 17-ethinylestradiol ($EE_2$) and Bovine somatotrophin (BST). Six weeks old F344 rats were divided into five groups and given an intraperitoneally injection of 200 mg of diethylnitrosamine (DENA). At two week after beginnig of experiment, DES, $EE_2$, BST. Phenobarbital were administered to group 1, 2, 3 and 4 respectively, group 4 is positive control and group 5 is negative control. At the same time, all groups received a single i.p. injection of D-galactosamine at a dose of 300 mg/kg and underwent 2/3 partial hepatectomy at week 5. All rats were sacrificed at the end of week 8 for assessment of liver lesion development. The liver was processed for immunohistochmical staining for GST-P and quantitatively analyzed by image analyzer. It was concluded that two synthetic estrogen hormones (DES, $EE_2$) was different significantly (p < 0.01) but BST was not different as compared with control group. Therefore, we thought that DES, $EE_2$ was promoting effects and BST was not in rat hepatocarcinogenesis.