The purpose of this study is to record continuously electrocardiograms of alpinists during different activities practiced in mountaineering, compare heart rate and QT interval at high altitude with those at sea level, and compare alpinists with nonalpinists. Analysis was attempted on 14 alpinists (9 male, 5 female, ages 26-45) to determine changes in heart rate and QT interval using continuous ambulatory electrocardiograms recorded at sea level and high altitude. Between 1983 and 1984, 9 of 14 alpinists (6 male, 3 female) were subjected to the study at high altitude, that is, at Mt. Kangchenjunga (Himalaya), Mt. Satopanth (Himalaya), and Mt. Jitudake (Butan), 4400 to 7800 m (mean 5710 m). The following were noted: Heart rate at high altitude was significantly higher both in daytime and nighttime. The circadian rhythm of the heart rate disappeared at extremely high altitude in several alpinists. A high correlation was noted between measured QT interval (QTm) and RR interval (r = 0.81, p = 0.005). Nighttime QTm at high altitude was prolonged in comparison with that of daytime so far as the RR interval remained the same. At high altitude, the nighttime corrected QT interval (QTc) was also significantly prolonged in spite of shortened RR interval. The mechanism of QTc prolongation is not clear. Many factors may impact on the QT interval during mountaineering.