The expression of aldolase A and B mRNAs during azo-dye-induced carcinogenesis in rat liver was examined. After feeding the dye for 18 weeks, the level of aldolase A mRNA increased to about 11 times that in a normal liver, with the concomitant decrease of aldolase B mRNA level to about 25% of that in a normal liver. These changes did not occur progressively during the carcinogenesis, but occurred as an additional phase after 4 week-feeding of the azo-dye. At this stage, the levels of aldolase A and B mRNAs were about 7 times and 45% of that in a normal liver, respectively. This biphasic pattern in the aldolase isozyme expression in the azo-dye-fed rat liver is discussed together with the kinetic data of the enzyme activity.