With the portable spirograph CORTEX X1 both the oxygen consumption and the carbon dioxide output can be determined. Therefore, the aim of the present study was to determine the accuracy of the CORTEX X1 in measuring F(E)O2, F(E)CO2 and V(E) when attached to a motor-driven mechanical syringe and to validate the CORTEX X1 against a standardized breath-by-breath system during a graded bicycle ergometry. Fifteen subjects (8 male, 7 female; 26.7+/-3.3 years) performed two graded exercise tests on a bicycle ergometer (50 W incline every 3 min) until volitional fatigue in randomized order. During rest and during the last 30 s of each step ventilatory and gas exchange parameters were measured with the CORTEX X1 and the OXYCONgamma. At rest and at each step no significant differences exist for VO2 (F = 0.97) and VCO2 (F = 0.90). The orthogonal regression equation of the VO2-values was VO2(X1) = -75.5 + 1.01 x VO2(Oxy) and the equation of the VCO2-values was VCO2(X1) = 21.7-1.008 x VCO2(Oxy). The VO2-max-values were nearly the same: 3569+/-924 ml/min (Oxy) and 3497+/-993ml/min (X1). Similar findings were made with regard to VCO2max 4117+/-1010 ml/min (Oxy) and 4126+/-1090 ml/ min. (X1). Maximal values for heart rate were 181+/-10 beats/ min (X1) and 180+/-8 beats/min (Oxy) (F=0.21), for maximal power 256+/-64 W (X1) and 257+/-63 (Oxy) (F = 0.0001) and for maximal ventilation 118+/-31 l/min) and 120+/-35 l/min (Oxy) (F = 0.03) with no significant difference. When attached to the motor-driven syringe V(E) was accurately measured up to 288 l/ min. Over a period of 40 min there was no drift observed in F(E)O2 and F(E)CO2. In conclusion, with the CORTEX X1 VO2 and VCO2 can be accurately determined.