  We present analytical and numerical studies of magnetorotational
instabilities occuring in magnetized accretion disks. In these studies we make
use of the linearised compressible MHD equations. These calculations are
performed for general radially stratified disks in the cylindrical limit. In
particular, we investigate the influence of nonvanishing toroidal magnetic
field component on the growth rate and oscillation frequency of
magnetorotational instabilities in Keplerian disks. We find the persistence of
these instabilities in accretion disks close to equipartition. Our calculations
show that these eigenmodes become overstable (complex eigenvalue), due to the
presence of a toroidal magnetic field component, while their growth rate
reduces slightly. Furthermore, we demonstrate the presence of
magneto-rotational overstabilities in weakly magnetized sub-Keplerian rotating
disks. We show that the growth rate scales with the rotation frequency of the
disk. These eigenmodes also have a nonzero oscillation frequency, due to the
presence of the dominant toroidal magnetic field component. The overstable
character of the MRI increases as the rotation frequency of the disk decreases.
