1. In rabbits transient and also permanent enlargements of the heart occur as the result of the intravenous injection of adrenalin, spartein and adrenalin, diphtheria toxin, and streptococci.
2. Transient and permanent enlargements of the heart to the left are associated with decrease in the size of the upwardly directed waves of the Q, R, S group in Lead II of the electrocardiograms and increase in the downwardly directed. Transient enlargement to the right is associated with changes in the opposite direction.
3. The stages in the process of enlargement have been observed and are associated with gradual changes in the size of these waves.
4. A diminution in the size of all the waves of the Q, R, S group was observed in degenerative conditions of the heart muscle.
5. Extreme degeneration of the structures of the heart was associated with slowing of the heart's action, lengthening of conduction time (P-Q), and lengthening of the time occupied by the individual waves of the electrocardiogram.
6. Ectopic ventricular contractions were seen in rabbits injected intravenously with suspensions of living streptococci.