PURPOSE: To discern whether arrhythmogenesis at high-altitude (HA) may differ depending on ascent or descent, as well as on age. METHODS: Male subjects (37.9+/-12.0 SD y, n=33) were separated into a young (Y) group (29.6+/-5.73 SD y, n=18) and an older (O) one (47.9+/-9.83 SD y, n=15). All subjects were monitored by Holter electrocardiography while successively ascending (41.2+/-7.51 SD min) and descending (38.7+/-6.68 SD min) between 2950 and 5050 m as car passengers on a 25 km road in Northern Chile. Arrhythmic events (AE) ensued when the difference between two consecutive RR intervals exceeded 0.16 sec. RESULTS: From 311 AE registered, 29% occurred on ascent and 71% on descent, the sinusal type predominating in both age groups. AE incidence, RR interval duration, and heart rate variability (HRV) in the time domain (RMSSD) increased during descent, as compared to ascent, in the Y group (p<0.05), but not in the O one. Independently of age, AE incidence along descent associates with the time previously spent at 5050 m (p<0.001). CONCLUSIONS: Rapid transitions at HA favor arrhythmogenesis, the latter becoming evident particularly in the Y group on descent. Age-dependent changes of autonomic activity appear to be involved in arrhythmogenesis on transitions at HA.