  Based on a simple, but fairly successful, model of the chemical evolution of
the Milky Way disk, we study the evolution of the abundances of the elements
He, C, N, O, Ne, Mg, Al, Si, S, Ar and Fe. We use metallicity dependent yields
for massive stars with and without mass loss. We find that most observed
abundance profiles are correctly reproduced by massive star yields, but C and N
require supplementary sources. We argue that massive, mass losing stars can
totally account for the abundance profile of C, while intermediate mass stars
are the main source of N; in both cases, some conflict with corresponding data
on extragalactic HII regions arises, at least if current observations in the
Galaxy are taken at face value. The observed behaviour of Al is marginally
compatible with current massive star yields, which probably overestimate the
``odd-even'' effect. We also find that the adopted ``inside-out'' formation
scheme for the Milky Way disk produce abundance profiles steeper in the past.
The corresponding abundance scatter is smaller in the inner disk than in the
outer regions for a given interval of Galactic age.

Astrophysics