Using three-dimensional non-equilibrium ionization ( NEI ) hydrodynamical simulation of the interstellar medium ( ISM ) , we study the electron density , n _ { e } , in the Galactic disk and compare it with the values derived from dispersion measures towards pulsars with known distances located up to 200 pc on either side of the Galactic midplane . The simulation results , consistent with observations , can be summarized as follows : ( i ) the DMs in the simulated disk lie between the maximum and minimum observed values , ( ii ) the \log \langle n _ { e } \rangle derived from lines of sight crossing the simulated disk follows a Gaussian distribution centered at \mu = -1.4 with a dispersion \sigma = 0.21 , thus , the Galactic midplane \langle n _ { e } \rangle = 0.04 \pm 0.01 cm ^ { -3 } , ( iii ) the highest electron concentration by mass ( up to 80 % ) is in the thermally unstable regime ( 200 < \mbox { T } < 10 ^ { 3.9 } K ) , ( iv ) the volume occupation fraction of the warm ionized medium is 4.9-6 % , and ( v ) the electrons have a clumpy distribution along the lines of sight .