The present study aimed to show that the stiffness and phase frequency responses obtained from sinusoidal and step length changes have similar form in guinea pig taenia coli smooth muscles. Sinusoidal length changes at 20 discrete frequencies in the range of 0.0007-1Hz or step length changes with a rise time of 60ms were applied during plateau level of the tetanic contraction (60mM KCl) and the force responses were recorded. Similar experiments were performed on the passive muscle with 0.1mg/mL verapamil. The stiffness and phase responses for tetanic contraction and passive muscle were obtained by using Fourier transformed results of force and length. Then the frequency response of the passive muscle was subtracted from that obtained for tetanic contraction. The general profiles of the stiffness and phase responses were similar for the two types of perturbations. The minimum stiffness occurred at 0.0012+/-0.0005Hz for sinusoidal perturbation and at 0.0022+/-0.0009Hz for step length perturbation. The minimum stiffness value was 3.79+/-0.59mN/mm (n=10) for sinusoidal perturbation and 4.02+/-1.18mN/mm (n=10) for step perturbation. The minimum phase angle was -30.2+/-5.5 degrees for sinusoidal perturbation and -28.2+/-11.6 degrees for step perturbation. In conclusion, results showed that step length perturbations instead of sinusoidal length perturbations could be used to find the frequency characteristics of smooth muscles. The results also indicate that 0.001-0.002Hz corresponds to the cycling rate of overall processes for the stretch-induced contraction mechanism in taenia coli smooth muscle contracted with high K, including the depolarization of the plasma membrane, calcium influx, phosphorylation of light chain, and cross-bridge action.