  We investigate the behavior of the magnetic pressure, $b^2$, in fully
turbulent MHD flows in ``1+2/3'' dimensions by means of its effect on the
probability density function (PDF) of the density field. We start by reviewing
our previous results for general polytropic flows, according to which the value
of the polytropic exponent $\gamma$ determines the functional shape of the PDF.
A lognormal density PDF appears in the isothermal ($\gamma=1$) case, but a
power-law tail at either large or small densities appears for large Mach
numbers when $\gamma >1$ and $\gamma < 1$, respectively. In the isothermal
magnetic case, the relevant parameter is the field fluctuation amplitude,
$\dbb$. A lognormal PDF still appears for small field fluctuations (generally
the case for {\it large mean fields}), but a significant low-density excess
appears at large fluctuation amplitudes ({\it weak mean fields}), similar to
the behavior at $\gamma > 1$ of polytropic flows. We interpret these results in
terms of simple nonlinear MHD waves, for which the magnetic pressure behaves
linearly with the density in the case of the slow mode, and quadratically in
the case of the fast wave. Finally, we discuss some implications of these
results, in particular the fact that the effect of the magnetic field in
modifying the PDF is strongest when the mean field is weak.
