  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 <n_e> 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 <n_e>=0.04\pm 0.01$
cm$^{-3}$, (iii) the highest electron concentration by mass (up to 80%) is in
the thermally unstable regime (200<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.
