The radiative core of the Sun rotates uniformly as suggested by helioseismological observations. It also rotates as slow as the average surface rotation. The pre-main squence Sun probably rotated much faster. If surface breaking has slowed down the Sun, a fast angular-momentum transport would be necessary to reduce the rotation rate of the core, too. Pure microscopic viscosiy is not sufficient. The problem of slow-down and equalizing of the core rotation is attributed to the magneto-rotational instability. Time-scales of 10-100 mill yr are found. A similar scenario may be possible in the radiative envelopes of Ap stars. Given their shorter life-times, the redistribution of angular momentum must be an ongoing process.