Using open-magnitude scaling, six normal subjects estimated the perceived magnitude of a range of added elastic loads (20–76 cmH2O/l), applied for a sequence of five breaths, at frequencies varying from 5 to 26.4 breaths/min. Two experiments were performed. In the first, frequency was increased by a reduction in expiratory duration (TE), and the duty cycle (ratio of inspiratory duration to total breath duration, TI/TT) ranged between 0.10 and 0.52. The perceived magnitude psi increased significantly with the peak airway pressure (Pm) (P less than 0.0001) but did not reach conventional significance with frequency (fb) (P = 0.15): psi = K0Pm1.23fb0.07 (r = 0.911). However, the sensory magnitude increased significantly as the duty cycle increased (P less than 0.01), but when it was included, the magnitude decreased minimally with frequency (P less than 0.01): psi = K0Pm1.3fb-0.97 TI/TT1.14 (r = 0.92). In the second experiment the duty cycle (TI/TT) was kept constant [(0.43 +/- 0.008 (SE)] and frequency (5–26.4 breaths/min) increased at the expense of shortening both TI and TE. The perceived magnitude of the added elastances decreased with the increase in frequency. However, when the perceived magnitude was corrected for the duration of inspiration, which is known to increase the sensory magnitude, psi = K0Pm1.3TI0.56, the sensory magnitude increased significantly with frequency (P less than 0.001): psi/TI0.56 = K0Pm1.21fb0.28 (r = 0.773). The decrease in inspiratory duration had a greater quantitative effect decreasing sensory magnitude than frequency had on increasing the magnitude. The effect of increasing frequency is complex and depends on the simultaneous intensity, duration of inspiratory pressure, and the duty cycle.