  New HI observations of Messier 31 (M31) obtained with the Effelsberg and
Green Bank 100-m telescopes make it possible to measure the rotation curve of
that galaxy out to ~35 kpc. Between 20 and 35 kpc, the rotation curve is nearly
flat at a velocity of ~226 km/s. A model of the mass distribution shows that at
the last observed velocity point, the minimum dark-to-luminous mass ratio is
\~0.5 for a total mass of 3.4 10^11 Msol at R < 35 kpc. This can be compared to
the estimated MW mass of 4.9 10^11 Msol for R < 50 kpc.
