A novel technique for remote determination of breathing frequency in unrestrained rodents using microwave radiation is described. Single mice were placed inside a rectangular waveguide operating at 2450 MHz. Because mice efficiently absorb radio frequency energy at 2450 MHz, any change in their absorption, as occurs rhythmically with breathing, can be detected by monitoring the changes in power transmitted through the waveguide. When the volume of the mouse increases during inspiration, transmitted power through the waveguide momentarily decreases - during expiration the reverse takes place.