  We compare the current effects of rotation in stellar evolution to those of
the magnetic field created by the Tayler instability. In stellar regions, where
magnetic field can be generated by the dynamo due to differential rotation
(Spruit 2002), we find that the growth rate of the magnetic instability is much
faster than for the thermal instability. Thus, meridional circulation is
negligible with respect to the magnetic fields, both for the transport of
angular momentum and of chemical elements. Also, the horizontal coupling by the
magnetic field, which reaches values of a few $10^5$ G, is much more important
than the effects of the horizontal turbulence. The field, however, is not
sufficient to distort the shape of the equipotentials. We impose the condition
that the energy of the magnetic field created by the Tayler--Spruit dynamo
cannot be larger than the energy excess present in the differential rotation.
This leads to a criterion for the existence of the magnetic field in stellar
interiors. Numerical tests are made in a rotating star model of 15 M$_{\odot}$
rotating with an initial velocity of 300 km$\cdot$s$^{-1}$.
