  We model the mass distributions of 40 high surface brightness spiral galaxies
inside their optical radii, deriving parameters of mass models by matching the
predicted velocities to observed velocity maps. We use constant mass-to-light
disk and bulge models, and we have tried fits with no halo and with three
different halo density profiles. The data require a halo in most, but not all,
cases, while in others the best fit occurs with negligible mass in the luminous
component, which we regard as unphysical. All three adopted halo profiles lead
to fits of about the same quality, and our data therefore do not constrain the
functional form of the halo profile. The halo parameters display large
degeneracies for two of the three adopted halo functions, but the separate
luminous and dark masses are better constrained. However, the fitted disk and
halo masses vary substantially between the adopted halo models, indicating that
even high quality 2-D optical velocity maps do not provide significant
constraints on the dark matter content of a galaxy. We demonstrate that data
from longslit observations are likely to provide still weaker constraints. We
conclude that additional information is needed in order to constrain the
separate disk and halo masses in a galaxy.
