We review some of the mixing processes that may influence the evolutio
n of massive stars, such as penetrative convection, and put our emphas
is on those which occur in the radiative envelope. There the main tran
sport mechanisms are a thermally driven meridian circulation, which de
parts significantly from the classical Eddington-Sweet description, to
gether with turbulent motions generated by the differential rotation.
This rotation-induced mixing will surround the convective core of such
stars with a region of decreasing helium content, which may prevent s
emi-convection from ever appearing; the extent of this region depends
sensitively on the rotation rate. The results are compared with those
obtained earlier by Mestel (1953). Work is in progress to verify wheth
er stars which rotate sufficiently fast may be thoroughly mixed.