Tyrosine aminotransferase activity (TAT) and its expression were measured in rats of different ages (3, 12 and 24 months). The enzyme activity showed a circadian rhythm with a peak at midnight due to different levels of transcription during the day. The circadian rhythm was present at all ages studied but showed some differences: an age-related shift of the peak was more evident in the actual levels of transcript. Both specific mRNA and enzyme activity analyses provide better insight into the complex modifications in gene-expression of aging animals.