This study aims at describing the workload demands during mountain bike races using direct power measurements, and to compare these data to power output and physiological findings from laboratory exercise tests. Power output (P, Watt) from 11 national team cyclists (9 male, 2 female) was registered continuously during 15 races using mobile crank dynamometers (SRM System). To evaluate the intensity of racing, incremental exercise tests with determination of P at aerobic and anaerobic thresholds (AT, IAT) and at exhaustion (MAX) were performed. Intensity zones were determined (zone 1 < AT; AT < zone 2 < IAT; IAT < zone 3 < MAX; zone 4 > MAX) and time spent during racing in these zones was calculated. Based on power output measurements P during racing was 246 +/- 12 W (male) and 193 +/- 1 W (female). P showed high variation throughout the race. In contrast heart rate (HR) was relatively stable during racing (male 177 +/- 6 bpm, female 172 +/- 7 bpm). 39 +/- 6 % of race time were spent in zone 1, 19 +/- 6 % in zone 2, 20 +/- 3 % in zone 3 and 22 +/- 6 % in zone 4. MTB races are characterized by a high oscillation in P with permanently elevated HR. A highly developed aerobic and anaerobic system is needed to sustain the high variation in workload.