An implantable telemetry transmitter is described for monitoring respiratory frequency and heart rate in animal physiological studies. The ECG is transmitted directly while the respiratory signal, derived from the temperature variation of the air in the respiratory tract, modulates the frequency of a subcarrier oscillator. A magnetic switch allows the implanted device to be switched on and off remotely. Details are given of the high packing density achieved by a modified cordwood method of construction. This system is being used to investigate the changes in heart rate and respiratory frequency associated with spontaneous diving in ducks, and typical results are presented.