A radiotelemetry system has been developed which is capable of transmitting electrocardiogram signals from domestic fowl. The transmitter circuit consists of a frequency-modulated Colpitts oscillator operating at 104·5 MHz, followed by a single-ended amplifier. The devices are fabricated as thin-film hybrid microcircuits. Transmitters were encapsulated in silicone rubber and implanted subcutaneously over the pectoral muscles in domestic fowl. Several sites for electrode placement were tried. The most successful yielded a large distinct QRS complex, which could be used as a trigger in measuring heart rate, and a good ratio of usable to unusable trace. These results were obtained when the electrodes were sutured 60 mm apart to the connective tissue covering the keel bone. Implantation of the devices did not affect the behaviour of the birds and there were no pathological lesions associated with them up to four weeks later.