The impedance of a positive column has been calculatedat medium pressures i.e. at the regime of stepionization-ambipolar diffusion. Calculations were carried out fortwo cases: (a) metastable level decay by electron impact; (b) metastable level decay on the walls as a result ofdiffusion. In both cases the impedance has generally an inductive character. Inthe case of metastable level decay by electron impact, theimpedance curve starts from the coordinate origin at $\omega =0 $ .At low frequencies the PC has a negatron property, while thestatic volt-ampere characteristics of the stationary PC becomesparallel to the current axis. The module |Z| reaches itsmaximum value at frequencies of about the order of $\sqrt{\tau_a^{-1}\alpha _{m}n_{0}}$ . At very high frequencies $\omega\longrightarrow \infty $ , the impedance reaches the real value of $Z=R_{0}$ .In the case when metastablelevels decay as a result of diffusion, the impedance curve startsfrom the negative real values at $\omega=0$ . The volt-amperecharacteristics of the stationary PC drops. The module |Z|reaches its maximum value at frequencies of about the order of $\sqrt{\tau_a^{-1} \tau^{-1}_{m}}$ . At very high frequencies $\omega \longrightarrow \infty $ , the impedance reaches the realvalue of $Z=R_{0}$ . In both cases, impedance curves have singlestructured shapes in spite of existence of several characteristictime scales of different order.