A three-channel frequency-modulation telemetry system for the noninvasive measurement of cardiac output, rate of respiration, e.c.g. and basal transthoracic admittance of the unconstrained human subject is described. Tetrapolar admittance plethysmography has been employed to obtain the transthoracic admittance and its variations corresponding to the cardiac cycle and respiratory changes. Pulse duration modulation has been used in the transmitter system. The nonlinearity of the system is less than ±2·0%. The telemetry transmitter has a range of about 100 m, it weights approximately 200 g and is contained in a box measuring 150×100×50 mm. The batteries used provide a continuous life of 20 h. The system has been used to obtain data from normal subjects in different postures with breath hold and during normal respiration.