The time courses of activities of aldolase, glyceraldehyde-3-phosphate dehydrogenase, hexokinase and pyruvate kinase were determined in stimulated rat thymocytes at 24 h intervals during a period of 72 h of culture. In parallel the mRNA levels of these enzymes were analysed by Northern blotting with specific probes. Both the enzyme activities and the corresponding mRNA levels reached their maxima 48 h after stimulation coinciding with the S-phase of the cell cycle. The isozyme types of aldolase and hexokinase in resting and in mitogen-stimulated rat thymocytes were identified by Northern blot hybridisation using isozyme-specific probes. In these cells the aldolase A is expressed, whereas type B and C could not be detected. The transcription of the aldolase A gene can be regulated by two different promoters. Depending on the alternative usage of the promoters the aldolase A-specific mRNA either contains the non-translated exons M1 or AH1. In rat thymocytes the promoter proximal to the exon AH1 is used while the expression of mRNA I, the type characteristic for muscle tissue, was not observed. In contrast to aldolase two isozyme types of hexokinase were detected. Hexokinase I as well as hexokinase II were present in thymocytes whereas hexokinase III was not detectable. A shift in the isozyme pattern was not observed during the cell cycle progression.