The expression of p21 ras and p110 myc oncogene proteins was examined in formalin-fixed paraffin-embedded sections of male Sprague-Dawley rat liver at various times after liver regeneration was induced by either a necrogenic dose of CCL4 or a 2/3 partial hepatectomy. Oncogene expression was detected with immunohistochemical (IHC) procedures in which either specific antibody for p21 ras or p110 myc was incubated with tissue sections and sites of antibody reaction visualized with an immunoperoxidase method using diaminobenzidine (DAB). The principle results of the study were as follows: (1) an increased p21 ras and p110 myc expression was detected in the tissue sections of regenerating liver at times comparable to those reported by others using either Northern or Western blot methods; (2) the increased oncogene expression was detected only in hepatocytes and occurred primarily in zones of the liver lobules known from previous studies to contain proliferating hepatocytes; and (3) the intensity of hepatocyte p21 ras or p110 myc staining did not increase greatly during regeneration but rather, a greater number of hepatocytes were positive for the oncogene proteins at times corresponding to increased oncogene expression in the Northern and Western blot methods.