  We address the problem of the influence of large-scale magnetic fields on
galactic rotation for the example of the spiral galaxy NGC 891. Based on its
rotation curve and the surface density of HI we determine, in the framework of
the global disc model, the surface density of matter. Then, based on the
surface brightness, we determine the corresponding profile of the local
mass-to-light ratio. We also model the vertical gradient of azimuthal velocity
in the quasi-circular-orbit approximation, and compare it with measurements. We
discuss what factors may influence the rotation of matter in NGC 891 and how
this can translate to changes in the profile of the local mass-to-light ratio.
In particular, we discuss the possible effect of magnetic fields on the motion
of ionized gas, and, consequently, on the determination of the profile of the
local mass-to-light ratio. Finally, we put forward the hypothesis that the
asymmetry in magnetic fields observed in NGC 891 might be responsible for the
observed anomalous behaviour of the vertical gradient.
