The acetylcholine receptor of the Torpedo electric organ acts as substrate for the enzyme protein carboxyl methyltransferase within receptor-enriched membranes and after it is purified. In right-side-out receptor-enriched vesicles, protein of the membrane. All four receptor subunits were found to be methylated from the inside as well outside of the membrane vesicles. The higher molecular weight subunits were found to be methylated to a greater degree than the lower molecular weight subunits.