The location of cholinesterase and non-specific esterase in the thyroid gland of the guniea-pig was studied with the light microscope. It was found that the idoxyl method for non-specific esterase activity under special conditions is superior to the cholinesterase method in a number of respects for the demonstration of the intra-, inter- and parafollicular cells. When using the indoxyl method the incubation period can be reduced from 2.5-3 hr to 40-50 min. Further, the reaction can be followed during the incubation. False localization of the reaction products is avoided,and nerves and erythrocytes are not stained. By varying the fixation time and the time of storage in gum arabic-sucrose, it was found that the miscellaneous activity of non-specific esterase in APUD cells (C-cells) and follicle cells may be due to both factors. In fresh tissue the activity of the enzyme was equal in follicle and C cells. Special cyst-like structures containing an esterase which is NaF-resistanct when alpha-naphthyl acetate is employed as a substrate and which gives a strong reaction at low pH values when 5-bromo indoxyl acetate is the substrate, are described, and their nature and possible origin are discussed.