The action of various inhibitors and activators upon esterase activity in the thyroid epithelial cells is demonstrated. The agents used were triorthocresylphosphate (TOCP), parachloromercuribenzoate (PCMB), Arsanillic acid, p-nitrophenyl dimethyl carbamate and bis p-nitrophenyl phosphate. TOCP was found to inhibit selectively the activity in the follicle cells proper when alpha naphthyl acetate was used as a substrate. Arsanillic acid (0,001 M) activated the follicle cells proper selectively, but if the concentration was raised to 0,01 M the effect was that of inhibition while the activity in the para-, inter- and intrafollicular cells was unchanged. The results obtained are related to previous biochemical and histochemical observations and the nature of esterases in the thyroid is discussed.