To investigate the effect of hyperoxia on EKG findings and to evaluate the applicability of EKG as noninvasive monitoring index of oxygen toxicity, 38 rabbits were continuously exposed to 6 different conditions-3 hyperbaric oxygenations (HBO-2.5, 3.5 and 5ATA, 100% $O_2$), normobaric oxygenation (NBO,100% $O_2$), hyperbaric aeration (HBA-5ATA, 21% $O_2$) and normobaric aeration (NBA, 21% $O_2$)-for 120 minutes and their EKG and time to dyspnea and convulsion were recorded. Dyspnea and death were observed in exposure conditions of HBO-3.5 and HBO-5 (Positive rate of dyspnea 10%, 100%, death : 10%, 25%, respectively) only, and convulsion in 4 oxygenation groups (NBO;20%, HBO-2.5;20%, HBO-3.5;20%, HBO-5;88%). Abnormal EKG findings included arrhythmia and ST-T changes and the incidences was increasing with doses(partial pressure of oxygen). In addition to EKG change, findings observed during exposure were dyspnea and convulsion in the order of appearance and when non specific ST-T change was accepted as positive(abnormal) finding, the frequency of abnormal EKG was statistically significant(p<0.01), but when it was excluded from positive results, the frequency of EKG change was not significant(p>0.05). These results suggest that the effect of hyperoxia on heart is myocardial ischemia and arrhythmia, that oxygenation more than 3.5ATA causes myocardial damage in 120 minutes exposure, and that EKG is valuable as monitoring index of oxygen toxicity.