Explants of adrenal medullae were cultured in defined media for up to 22 hr, during which time the tissue remained histologically intact. Addition of N6-methyladenosine to the medium led to a diminution in the activity of tyrosine hydroxylase (EC 1.14.16.2) in the tissue. The enzyme activity was inversely proportional to the concentration of N6-methyladenosine in the culture medium. The extent of loss of tyrosine hydroxylase, as measured by immunochemical titration, corresponded to the degree of loss in enzyme activity under the same conditions. The decreased amount of enzyme protein was due to a decreased rate of synthesis of tyrosine hydroxylase. A significant decrease in the relative rate of tyrosine hydroxylase synthesis indicates the selectivity of this effect of N6-methyladenosine. The rate of enzyme degradation was not affected by this compound. Neither adenosine, N6-cyclohexyladenosine, nor several other methylated nucleosides including N1-methyladenosine, N7-methylguanosine and N2-methylguanosine had an effect on the enzyme. However, two other N6-substituted adenosines, N6-dimethyladenosine and N6-gamma gamma-dimethylallyladenosine, were effective in reducing tyrosine hydroxylase. The results are consistent with the view that specific substitutions at the N6 position of adenosine could play a role in regulation of levels of tyrosine hydroxylase by altering its rate of biosynthesis.