Development of ECG interval durations in maturing FVB/N neonates. Electrocardiograms were recorded for twenty mice assayed longitudinally from PND4 to 9 weeks of age. A) Heart rate of male and female mice throughout development. A strong interaction effect (P ≤ 0.001) between age and sex is present during development. (*P ≤ 0.05, linear mixed model, n = 8 females and 12 males). Arrow denotes point at which mice were weaned. Inset in (A) illustrates the linear nature of heart rate development in young neonates, with best fit linear regression of combined male and female heart rate data from seventeen neonates collected longitudinally every day from PND2 to PND14. The average slope of individuals is 22 ± 4 bpm per day, with R2 = 0.85 ± 0.11. B) PR interval during development. No interaction effect is present over development, however isolated analysis of the 9 week time point confirms a sex based difference is present in adults. (*P ≤ 0.05, student's t-test) C) QRS interval, and D) corrected QT interval (QTc) are presented as mean ± SEM, with no significant differences or interaction effects present.