PROLONGED UNINTERRUPTED recordings of physiologic data in man has been difficult to obtain in the past because of the physical boundary imposed on the patients' activities by recording equipment. Advances in electronic miniaturization have led to the development of transistorized radio instruments which make it possible to signal information in a manner unencumbered by bulky recording equipment. The purpose of tin's preliminary report is to briefly describe one technique being used in our laboratory to continuously record the heart rate over several hours' time. The initial application of this technique has been in the observation of the sleeping pulse rate. Method The system of radiotelemetry recording can best be described by following the signal from the patient to the final recording. The transmitter operates on a frequency allocated by the Federal Communications Commission (Fig. 1), and is roughly the size of a cigarette package (8 × 8 cm.) and weighs