The impedance of a positive column has been calculated at medium pressures i.e. at the regime of step ionization-ambipolar diffusion. Calculations were carried out for two cases: (a) metastable level decay by electron impact; (b) metastable level decay on the walls as a result of diffusion. In both cases the impedance has generally an inductive character. In the case of metastable level decay by electron impact, the impedance curve starts from the coordinate origin at $\omega =0 $. At low frequencies the PC has a negatron property, while the static volt-ampere characteristics of the stationary PC becomes parallel to the current axis. The module |Z| reaches its maximum 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 metastable levels decay as a result of diffusion, the impedance curve starts from the negative real values at $\omega=0$. The volt-ampere characteristics 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 real value of $Z=R_{0}$. In both cases, impedance curves have single structured shapes in spite of existence of several characteristic time scales of different order.