The purpose of this work is to analyse changes in ECG and heart rate variability (HRV) metrics in athletes during 10 km running in mountain route conditions. Eighteen healthy athletes carrying a 12-lead ECG GE Holter recorder and a heart rate monitor ran a route with slopes similar as trail races. QRS duration, QRS area, R-wave peak, ST elevation at J-point and J+60 ms, ST slope and T-wave peak indices were computed after signal-averaging ECG segments at different sloping stages of running (S1-S6) and at a control stage (S0) before running. HRV analysis included standard time and frequency metrics: mean RR (normal, N) interval, SDNN, RMSSD, low and high frequency absolute and normalized power (LF, HF, LFn, HFn) and LF/HF ratio. QRS area and R-peak were reduced during stage 1 in lateral leads, comparing to rest before running. ST slope was significantly higher during upslope stages in leads II, V4 and V5. T-wave amplitude increased significantly in precordial leads during upload running. ST segment depressed in leads II, III and V5 respect control. Mean RR, SDNN, LF and LFn showed high significant differences (p<0.01) among stages and HF and LF/HF were also varying (p<0.05). Changes of ECG and HRV indices can help understanding the cardiac function in runners performing extreme stress.