A series of simple analytical models for the evolution a galactic disk have been constructed. General solutions can be obtained under the assumption of pure radial gas flows, a star formation rate proportional to the nth power of the surface gas density, a constant IMF, and the instantaneous recycling approximation. Models with small radial flow velocities in the range 0.05 to 0.1 km s–1 and an initial exponential surface mass density can reproduce, for galactocentric radii larger than about 5 kpc, the Galactic 16O abundance gradient and the present surface gas distribution.