The impact of age on the enzymatic activities of adenosine metabolic enzymes, i.e., adenosine deaminase, adenosine kinase, cytosolic- and ecto-5'-nucleotidase have been assessed in the brain sleep/wake regulatory areas of young, intermediate and old rats (2, 12 and 24 months, respectively). There were significant spatial differences in the distribution of enzymes of adenosine metabolism in the brain. Age did not impact on the enzymatic activity of adenosine deaminase. Adenosine kinase activity increased significantly in the cerebral cortex of old animals. However, there were no differences in the activity of adenosine kinase between young and intermediate aged rats. The largest age-related changes were in the activity of cytosolic- and ecto-5'-nucleotidase and there was a significant age-related increase in the activity of these enzymes in the sleep/wake regulatory areas. In addition, the activity of cytosolic- and ecto-5'-nucleotidase increased in the cerebral cortex of old and intermediate age rats when compared to young animals. An increase in the enzymatic activities in the cerebral cortex of adenosine kinase and 5'-nucleotideases was accompanied by an increase in the level of their mRNA. An increase in the activity of 5'-nucleotideases with age likely leads to an increase in adenosine levels in the brain.