PURPOSE: There are few data examining cardiovascular physiology throughout a marathon. This study was devised to characterize electrocardiographic activity continuously throughout a marathon. METHODS: Cardiac activity was recorded from 19 subjects wearing a Holter monitor during a marathon. The 19 subjects (14 men and 5 women) were aged 39 +/- 16 years (mean +/- SD) and completed a marathon in 4:32:16 +/- 1:23:35. Heart rate (HR), heart rate variability (HRV), T-wave amplitude, T-wave amplitude variability, and T-wave alternans (TWA) were evaluated continuously throughout the marathon. RESULTS: Averaged across all subjects, HRV, T-wave amplitude variability, and TWA increased throughout the marathon. Increased variability in T-wave amplitude occurred in 86 % of subjects, characterized by complex oscillatory patterns and TWA. Three min after the marathon, HR was elevated and HRV was suppressed relative to the pre-marathon state. CONCLUSION: HRV and T-wave amplitude variability, especially in the form of TWA, increase throughout a marathon. Increasing TWA as a marathon progresses likely represents a physiologic process as no arrhythmias or cardiac events were observed.