Changes in levels of serum total lactic acid dehydrogenase(LDH) and patterns of its isozyme were studied after a single intraperitoneal injection with polyaromatic hydrocarbons(3-methylcholanthrene, 3-MC, 100mg and 7, 12-dimethylbenz(a) anthracene, DMBA, 20mg) in Sprague-Dawley rats(female, 140-150 g). Serum LDH level of rat on day 5 after chemicals administration was markedly elevated when compared with the control value. Levels of LDH were produced a maximal value about 2 to 3 fold of control on day 15 in both 3-MC and DMBA treatment. There resulted in increase of the heart type subunit(LDH1) of isozyme patterns, which was accompanied by a relative decrease in percent of the muscle type subunit(LDH5). In the case of DMBA treatment, percent of LDH1 after 10 day decreased progressively during the period studied when compared with initial value. Level of rat serum total LDH was characterized by a sharp decrease on day 15 later in both DMBA and 3-MC administration and exhibited close value to control serum on day 20 or day 30. Although there was an increase or decrease in serum LDH level through the period observed, LDH5 showed lower percent than in control LDH level over 20 day or 30 day after chemicals treatment increased considerably and was about 2 to 4 fold above controls. Results of this investigation may, therefore, be that both the LDH level and pattern of isozyme exhibited characteristic changes in rat serum served as an useful basis for prognosis in cancer at various stages of development.