A system developed to expose chronically a large number of small animals to 918‐MHz microwave has been modified to permit exposure at 2450 MHz. Special transmitting and termination transducers were used to retrofit the exposure chambers of the 918‐MHz system. The use of separate cells that consist of cylindrical waveguides, which are excited by circularly polarized, guided waves, provides a relatively constant and easily quantifiable coupling of microwave energy to each animal, regardless of their position, posture, or movement. The VSWR in each cell is sufficiently low that any number of cells can be coupled to a single source through a power splitter without the need for an isola