Measurements on human brain samples of some enzymes concerned with neurotransmitter synthesis suggest serious losses with age. The most severe loss found was that in striatal tyrosine hydroxylase activity, the rate-controlling enzyme in the synthesis of dopamine. Cell counts in the substantia nigra where this dopaminergic tract originates suggest that the decrease in enzyme activity is partly due to cell loss, but must largely reflect decreased activity of residual cells. It is possible that this loss may account for some of the difficulties in movement seen in aged individuals and that it might be less if pigment formation in these cells could be inhibited.