The physiological and psychophysical effects of combined flow resistive loads were studied systematically by using the Multistage Evaluation Scale (MES), determining the number of Just Noticeable Difference (JND) steps, and constructing equivalent resistive-load-sensation contours. During combined resistive loading, the relative changes of inspiratory and expiratory peak flow rate (VImax/VEmax) were related to both the JND step number and the inspiratory versus expiratory resistance ratio (IR/ER). However, the ratio VImax/VEmax was related to the IR'/ER' ratio only. The subject's perception of combined loads differing both in intensity and IR/ER ratio could be well depicted by the equivalent resistive-load-sensation contours. Hence we could define a model for predicting the physiological and psychophysical effects of various resistive loading conditions.