A rat brain cDNA library was screened at low stringency with an aldolase B cDNA probe corresponding to the coding sequence of the mRNA, then at high stringency with a 3' non-coding aldolase A cDNA probe. One clone, which hybridized only under the first conditions, was further characterized and used to screen the library again. Two overlapping clones, complementary to aldolase C mRNA, were obtained. They cover the 113 carboxy-terminal coding residues and the 3' non-coding region up to the poly(A) tail. Their nucleotide sequence was determined. In the coding region the overall homology with aldolase A was 67% at the nucleotide level and 76% at the protein level. With aldolase B these values were 63% and 65% respectively. The 3' non-coding region was 380 bases long and did not exhibit any homology with the untranslated 3' extension of aldolase A and B mRNAs. Southern blot analysis indicates that probably a single aldolase C gene exists per haploid genome. Aldolase C mRNA was detected at low concentration in practically all the foetal tissues and its expression markedly and rapidly decreased after birth. In brain the concentration of aldolase C mRNA remained high and stable even after birth. Aldolase C mRNA is approximately 50-fold more abundant in brain than in foetal tissues, which are the richest in messenger RNA. In the course of azo-dye hepatocarcinogenesis the aldolase C gene is re-expressed early, with a maximum at the 4th week of carcinogenic diet, which probably corresponds to the maximal proliferation of the oval cells.