A dilute suspension of particles with permanent electric or magnetic dipole moment is subjected to an applied electric or magnetic field which suddenly changes its value. The relaxation of polarization or magnetization to the new equilibrium value is calculated from the solution of the Smoluchowski equation for the orientational distribution function. It is assumed that the new field has the same or opposite direction as the old one. For reversal of a strong field, comparison is made with an asymptotic calculation of van Kampen and with the solution of an approximate relaxation equation proposed by Martsenyuk