The effect of aging on the activity of tyrosine hydroxylase (TH) and on the number of TH-positive perikarya in the hypothalamus was studied in old and young female rats. The activity of TH in the mediobasal hypothalamus (MBH) of old rats was significantly (P less than 0.025) less than that in young rats. In old rats, the Km of TH for tyrosine as well as cofactor, 6-methyl-5,6,7,8-tetrahydropterine (6MPH4), was markedly greater than the Km in young rats. The maximal velocity was only slightly reduced in old animals. Contiguous coronal sections of the brain of an old and a young female rat were immunocytochemically stained for TH, and the TH-positive perikarya in the hypothalamus were counted. In the circumventricular region, 6793 TH-positive perikarya were present in the young brain and 6632 in the old brain. In the arcuate region, 2868 and 2760 TH-positive perikarya were counted in the young and old brain, respectively. It is concluded that the reduced TH activity in the MBH of old rats is not a consequence of a reduction in the number of TH-positive perikarya in the arcuate or circumventricular regions of the hypothalamus but is due to a reduction in the affinity of TH for its substrate and cofactor.