  A parameterized model of the mass distribution within the Milky Way is fitted
to the available observational constraints. The most important single parameter
is the ratio of the scale length R_d* of the stellar disk to R0. The disk and
bulge dominate v_c(R) at R<R0 only for R_d*/R0< 0.3. Since the only knowledge
we have of the halo derives from studies like the present one, we allow it to
contribute to the density at all radii. When allowed this freedom, however, the
halo causes changes in assumptions relating to R << R0 to affect profoundly the
structure of the best-fitting model at R >> R0. For example, changing the disk
slightly from an exponential surface-density profile significantly changes the
form of v_c(R) at R >> R0, where the disk makes a negligible contribution to
v_c. Moreover, minor changes in the constraints can cause the halo to develop a
deep hole at its centre that is not physically plausible. These problems call
into question the proposition that flat rotation curves arise because galaxies
have physically distinct halos rather than outwards-increasing mass-to-light
ratios.
  The mass distribution of the Galaxy and the relative importance of its
various components will remain very uncertain until more observational data can
be used to constrain mass models. Data that constrain the Galactic force field
at z > R and at R > R0 are especially important.
