Adopting the widely held view that plasma transport in rotating magnetospheres, like those of Jupiter and Saturn, proceeds through the interchange of magnetic flux tubes, we study the physical characteristics and stability properties of interchange motions. We start by examining a broader category of motions containing all the modes driven by buoyancy forces of gravitational or magnetic nature; these modes, known as quasi-interchanges, fall into two types: type 1, to which the familiar interchange mode belongs, and type 2, in which the plasma moves essentially along magnetic field lines. In the second part of the paper, we focus on the interchange mode, with the purpose of deriving formal analytical expressions for both its dispersion relation and its stability threshold. ¿ 2001 American Geophysical Union