Four modes of administrating atropine and neostigmine to reverse pancuronium block were studied in thirty-eight anesthetized patients, with special reference to their effects on heart rate and electrocardiogram. At the end of operation in group 1 a mixture of atropine (0. 5 mg) and neostigmine(1. 0 mg) was rapidly injected together. In group 2 the same mixture was slowly injected in 3 minutes. In group 3 atropine was rapidly injected and 2 minutes later neostigmine was rapidly injected. In group 4 neostigmine was slowly injected in 1 minute and then immediately atropine was rapidly injected. The results were as follows. 1) In all groups a transient increase in heart rate was followed by a decrease which continued over 30 minutes. 2) The changes in heart rate were most pronounced when atropine was given before neostigmin(group 3) On the other hand when atropine was given immediately after neostigmine the changes in heart rate seemed to be smallest. 3) A small number of cases showed transient ECG changes in each group. It is considered that one must be careful about arterial gas and ECG changes although the number of cases is small and certain correlation was not indicated but the ECG of the five of 38 patients are abnormal, four of these are among the abnormal cases in the gas study, two of the four abnormals are under PaO₂, 60 mmHg.