  The nearly face-on spiral galaxy NGC 3938 has been observed in the $H_\alpha$
line with the TAURUS II Fabry-Perot interferometer at the William Herschel
Telescope in order to study the kinematics of the ionized gas. We are able to
construct intensity, velocity and velocity dispersion maps for this galaxy. The
rotation curve of the galaxy is calculated up to 4.5 radial scale lengths from
the galactic centre. The residual velocity field shows very small values with
no systematic pattern. The mean velocity dispersion is approximately constant
with radius at about 11 km/s as previously reported for the neutral and
molecular gas. We have also studied the relation between intensity and velocity
dispersion for the ionized gas. We have found that this distribution is
compatible with a turbulent gas relaxing to a Kolmogorov type turbulence as the
stationary regime. The average dispersion varies with intensity as $\sigma
\propto I^{1/8}$ although it becomes much steeper at higher intensities, for
which the dispersion is kept almost constant at a value of about 19 km/s.
