  We present a chemical evolution model for M31 based on a pronounced
inside-out formation scenario. The model reproduces the three main
observational constraints of the M31 disk: the radial distributions of the
total baryonic mass, the gas mass, and the O/H abundance. The model shows good
agreement with the observed: SFR(r), Mstars(r), the C/H, N/H, Mg/H, Si/H, S/H,
Ar/H, Cr/H, Fe/H, and Z gradients. From reproducing the observed gas mass, we
find that the star formation efficiency is variable in space, for the whole
disk, and is constant in time for most of the evolution (t< 12.8 Gyr). From
reproducing the observed SFR, we find that the efficiency decreases almost to
zero for 12.8<t(Gyr)< 13.0 and r>12 kpc. All the predicted Xi/H(r) gradients
show three different slopes, due to the r-dependence of the the star formation
efficiency and the inside-out galactic formation.
