Present study was undertaken to elucidate possible distortion of phase response and amplitude response of various respiratory parameter such as VO2, VCO2 and VE to sinusoidal work load by comparing model analysis with manual analysis. Also, an attempt was made to determine whether there is any relationship between the characteristics of response of these parameters and the aerobic capacity of subjects. Six healthy male subjects were performed exercise on an electrically braked bicycle ergometer for 32 min. The work load was varied sinusoidally between 30 watts and 60% VO2max being under anaerobic threshold with periods from 1 to 16 min. These parameters were determined in breath-by-breath mode with a computer system and mass spectrometer. In model analysis, amplitude and phase responses were well described by first order exponential model, and strong correlations were observed between magnitude of phase response or time constant of amplitude response and aerobic capacity. Manual analysis revealed that respiratory responses to sinusoidal work load are not completely sinusoidal but somewhat distorted forming saw-tooth waves with steeper downslopes.