  Stellar rotation produces an internal mixing of the elements due to shear
instability and meridional circulation. This leads to observable $N/C$
enhancements in massive stars above about 7--9 $M_{\odot}$. Rotation also
favours mass loss by stellar winds. Mass loss effects dominate for masses above
30 $M_{\odot}$, while mixing dominates below that limit.
  The effects of mixing are also much larger at lower metallicity $Z$, because
the internal $\Omega$--gradients are steeper. This appears to be in agreement
with observations in the SMC.
  At very low $Z$ and Z=0, mixing between the He--burning core and the
H--burning shell leads to the production of primary N in intermediate mass
stars.
  Such enrichments increase the metallicity of the rotating star, also massive
Z=0 stars with moderate initial velocities currently reach break--up velocity
during a fraction of the MS phase. Both effects favour mass loss in Z=0 stars,
which have ejecta with abundance anomalies very similar to those of C--rich
very metal poor stars.
