Abstract: A microwave radiometer for the passive, noninvasive measurement of fluid content in animal bodies. The radiometer system includes a ridged waveguide antenna which is responsive to frequencies in the microwave range emitted from the patient's body and is of compact size and light in weight so as to be easily secured to the patient with minimum discomfort and loss of mobility. A heating system is attached to the antenna so as to maintain antenna temperature at approximately the temperature of the patient's body surface. The associated control circuitry processes signals received from the antenna at one or more frequencies and produces an output signal which correlates to the amount of fluid within the measured region of the body.