  The rotation curves of 20 spiral galaxies are examined in the light of a toy
model (Soares 1992) which has as the main feature the assignment of a high M/L
ratio (=30; Ho=50 km/s/Mpc) to the visible matter. The observed rotation of all
galaxies can be accommodated without the assumption of a dark halo.
  Moreover, the suggestion is made that the fact that almost all available
rotational velocity measurements are derived from emission lines emitted by
galaxian gas (either neutral or ionized) makes them inappropriate as tracers of
the galaxy gravitational potential. To account for that, the model introduces
an effective potential meant to describe the hydrodynamics inside a gaseous
disk. The general morphology of the curves (i.e., the presence of a plateau in
V(r) X r) is interpreted in this framework as a consequence of the
hydrodynamical characteristics of galaxian disks.
  The Tully-Fisher relation is expressed in terms of model parameters and used
as an additional constraint in the process of fitting the model to the observed
rotation of the galaxies.
