A study was conducted to examine the validity of the system dynamics (SD) model of expired gasdynamics during incremental tests, individually. In modeling, it was assumed that (1) the increment of exercise intensity follows the test time, (2) the behavior of oxygen uptake (VO_2) and lactate (La) depend on exercise intensity, (3) CO_2 output (VCO_2) is the sum of metabolic VCO_2 that stems from increased VO_2 and VCO_2 generated by buffering La and (4) ventilation (V_E) is the sum 0f ventilation and hyperventilation that depends on La. The validity of the model was examined by (a) determination coefficient (r^2) as a fit index of simulated V_E, VO_2, VCO_2 and La to measured them, (b) equivalence consisting of the correlation coefficient (r) and paired t test between simulated and measured data at the anaerobic threshold (AT) and respiratory compensation point (RCP). Seven male subjects performed two protocol types of incremental tests to exhaustion on a cycle ergometer (TEST I, TEST II). The measurements in TEST I were used for modeling, and TEST II measurements for establishing the validity criteria of the model. In all subjects, _the fit index of the simulated V_E, VO_2, VCO_2 and La was high (r^2 = more than 0.954. P<.05). Correlation coefficients of simulated VO_2, % VO_2max and Watt at AT and RCP were respectively high (P<.05), and mean differences between simulated and measured VO_2, % VO_2max and Watt at AT and RCP were not statistically significant. These results indicated that the SD model of expired gas dynamics during incremental testing was valid. In conclusion, it is suggested that individual aerobic fitness can be analytically evaluated using SD modeling with incremental test measures.