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Cholinesterase and succinyldehydrogenase activity of surgically removed left atrial auricles from patients in atrial fibrillation and in sinus rhythm have been compared, using histochemical methods. Higher cholinesterase and lower succinyldehydrogenase activity has been found in atrial fibrillation than in sinus rhythm. The pulmonary capillary mean pressure of patients with atrial fibrillation and sinus rhythm have been also compared. There was no significant difference between the two groups. On the basis of the reported examinations, it is not possible to decide whether the changes in enzyme activities are the cause or the result of atrialfibrillation.Studies on the histochemical structure and metabolism of various parts of the heart muscle have shown the existing basic differences in the intermediary metabolism of the conductive system of the heart and of the working muscle mass, respectively. The oxygen uptake of the conductive system is lower, similar to the succinyldehydrogenase activity, while the glycogen concentration is higher than that of the working musculature (Schiebler, Stark, and Caesar, I956; Isaacson and Boucek, I968; Opie, I969). In contrast to the oxidative metabolism of the working muscles, that of the conductive system is characterized by anaerobic glycolysis, and this difference becomes more obvious as we go up the scale from lower to higher stimu- These histochemical and ontogenetic characteristics led us to the following hypothesis. According to Nachmansohn (I962) the acetylcholine-cholinesterase system is regarded as the specific chemical structure to be foundin each zoological species and able to produce action potentials with propagation in the form of impulses. The necessary energy to supply this system is provided with the aid of anaerobic metabolism by the specific musculature (Opie, I969). It might be assumed that the origin of some arrhythmias lies in the pathological shift of the metabolism and of the acetylcholine-cholinesterase system. The development of ectopic foci producing stimuli or the formation of circus movement (re-entry and micro-re-entry mechanism) are possible mechanisms of arrhythmias. The function of these foci and pathways is similar to that of the normal conductive system. It seems plausible that similarity in the function involves similar histochemical processes. It is to be expected that in these cases of arrhythmias high cholinesterase activity and increased rate of anaerobic metabolism -normally present only in the conductive systemshould be demonstrable in the working muscles as well.As a first step to test the value of this hypothesis, cholinesterase and succinyldehydro-

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