ZnO/ZnMgO multiple quantum wells are investigated in terms of intersubband transitions within the conduction band. The Schrodinger's and Poisson's equations are solved self-consistently. The intersubband transition energy is represented as a function of their physical dimension and Mg concentration. The effects of the depolarization and excitonic shifts on the transition energy are considered. A high level of doping, such as 1 x 10(25) m(-3), is required to screen the built-in electric field, which is around 0.355 MV/cm at most. In case of screening, it is possible to have quantum wells aligned in energy and a homogeneous electron distribution over the quantum wells. The transition energy is between 51 and 403 meV if the width of the constituting quantum wells is between 20 and 100 angstrom. The contribution of the many-body effects is 29 meV at most. However, this contribution strongly depends on the well width: it can be as low as -1 meV.