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Summary. 1 Valsalva's experiment was carried out on 25 healthy subjects of whom 13 were smokers and 12 non-smokers before and after smoking of 1–3 cigarettes. In twelve of these subjects control experiments without smoking between two successive Valsalva experiments were carried out. 2 Smoking did not as a rule alter the resting heart rate in non-smokers, but increased it on an average for 16 beats/min. in smokers. 3 The total increase or decrease in heart rate during and after the strain was used as a measure of reactivity. It varied in different subjects, some showing no increase in heart rate at all. Occasionally the type of response obtained in a certain subject could be altered. The smoking tends to decrease the reactivity when effective. A significant correlation between heart rate response and alteration in the height of the P and T waves was found. No correlation between P-Q time and reactivity seemed to exist. 4 The mean electrical axis (and changes caused by strain and by smoking) was calculated with the help of the Dieuaide's chart. The strain tends to deflect the axis rightwards. Smoking did not as a rule affect the mean direction of the electrical field. In 5 instances, however, a leftward shift in the axis was noted. There was no correlation between the size of the heart rate reaction and the rightward deflection of the axis during the strain. 5 Certain special electrocardiogratphic findings will be treated in a separate paper.