By studying the action of drin compounds on isolated frog heart (Straub-Fuhner's method), the author obtained the following results: 1. In the case of dun compounds at a low concentration there occurs a slight decrease in the amplitude of heart beats. 2. At a high concentration, atrioventricular block and low amplitude are obserbed. Such changes are more prominent at a higher concentration, which eventually leads to diastolic cardiac arrest. 3. Premedicaticn of atropine does not reduce the suppressive action of dieldrin. 4. When atropine is given to the heart after exposure to dieldrin, an increase in the amplitude and continuance of heart beats may be seen. 5. Application of adrenaline previous to dieldrin, does not influence the decrease in the amplitude of heart beats. 6. When adrenaline is applied just prior to the cardiac arrest by dieldrin, heart beats are maintained with an increase of their amplitude. 7. It is concluded that drin compounds are not cholinesterase blockers such as alkylphosphates.