The ICH S7B guideline specifically requests the evaluation of the QT interval in in vivo models as an accepted risk factor for fatal tachyarrythmias. While it recommends correcting the QT interval for heart rate (HR), it also concedes that such corrections can yield misleading results. Data acquired from 40 cynomolgus monkeys (CM) and 66 Beagle dogs (BD) on 64 and 166 episodes, respectively, of 25-h ECG data collection in healthy control group animals were analyzed for this publication. The total number of ECGs evaluated was 10,761 (CM) and 24,882 (BD). The two species appear to have some difference in cardiac repolarization regulatory mechanisms. CM are more subject to diurnal fluctuations of autonomic nervous tone, which leads to dramatic variation of QT interval duration (up to 12.7%) at the same HR (60 to 70 bpm) in different light cycles. BD do not have such a variation. Different species require different QT correction formulas. Van de Water's correction provides optimal results in BD; Bazett's correction presents optimal results in CM. Fundamental behavioral differences (domesticated vs wild animals) may require individual approaches in the interpretation of the safety pharmacology studies in various species.