The purpose of the study was to measure the demands of off-road cycling via portable spirometry, leg-power output (PO), heart rate (HR) and blood lactate (BLa) concentration. Twenty-four male competitive cyclists (age: 29±7.2 yrs, height: 1.79 ± 0.05 m, body mass: 70.0 ± 4.9 kg, VO2peak: 64.9 ± 7.5 ml·kg(-1)·min(-1)) performed simulated mountain bike competitions (COMP) and laboratory tests (LabT). From LabT, we determined maximal workload and first and second ventilatory thresholds (VT1, VT2). A high-performance athlete (HPA) was used for comparison with three groups of subjects with different sport-specific performance levels. Load profiles of COMP were also investigated during uphill, flat and downhill cycling. During the COMP, athletes achieved a mean oxygen uptake (VO2COMP) of 57.0 ± 6.8 ml·kg(-1)·min(-1) vs. 71.1 ml·kg(-1)·min(-1) for the HPA. The POCOMP was 2.66±0.43 W·kg(-1) and 3.52 W·kg(-1) for the HPA. POCOMP, VO2COMP and HRCOMP were compared to corresponding variables at the VT2 of LabT. LabT variables correlated with racing time (RTCOMP) and POCOMP (p < 0.01 to <0.001; r-0.59 to -0.80). The VO2peak (LabT) accounted for 65% of variance of a single COMP test. VO2COMP, POCOMP and also endurance variables measured from LabTs were found as important determinants for cross-country performance. The high average VO2COMP indicates that a high aerobic capacity is a prerequisite for successful COMP. Findings derived from respiratory gas measures during COMPs might be useful when designing mountain bike specific training. Key pointsCross- country cycling is characterized by high oxygen costs due to the high muscle mass simultaneously working to fulfill the demands of this kind of sports.Heart rate and blood lactate concentration measures are not sensitive enough to assess the energy requirements of COMP. Therefore, respiratory gas and power output measures are helpful to provide new information to physiological profile of cross- country cycling.An excellent cycling-specific capacity is a prerequisite for successful off-road cycling.Data determined from LabT might be utilized to describe semi-specific abilities of MB- athletes on a cycle ergometer, while data originating from COMP might be useful when designing a mountain bike specific training.