Effects of general hypothermia on E. C. G. were studied on dogs. Thirteen dogs were cooled with ice bags following intravenous pentothal anesthesia supplemented with drop infusion of Cocktail lytique. Rectal temperature of dogs were kept at the range from 30°C to 27°C during the experiment and E. C. G. was recorded in several stages of cooing. In two of them E. C. G. were taken during a period of rewarming, too. The following alterations in E. C. G. were noticed. (1) R-R: R-R interval was gradually prolonged with progress of cooling. It prolonged in lineal fashion with the decrease in rectal temperature. On rewarming R-R interval returned to normal value. (2) P-Q: P-Q interval was slightly prolonged on cooling in most cases (84.6%) and returned to normal on rewarming. This change was not marked compared with prolongation of Q-T. (3) Q-T: Q-T interval markedly prolonged with the decrease in rectal temperature. This change disappered on rewarming. (4) QRS: Widening of QRS was seen on cooling in most cases. This change was marked when rectal temperature was decreased below 30°C, hewever, returned to normal on rewarming. Occasionally R' wave appeaerd during cooling in about one third of the series. It was anticipated that widening of QRS is due to occurrence of R' wave. (5) P. Q. S.: No marked changes were noticed in P. Q. and S. waves. (6) R: R wave increased its amplitude on cooling. (7) T: Inversion or diphasion of T wave appeared when rectal temperature decreased to around 30°C in about one half of the series and coronary T was observed about one third in leads II and III. (8) ST: ST depression occurred in 38.5% when the rectal temperature decreased below 30°C in leads II and III. (9) One dog died of ventricular fibrillation which occurred abruptly when rectal temperature was reduced to 26.3°C. Temporary ventricular premature beats appeared in another dog during rewarming.