A description is given of the development of systems for the control of body temperatures in small infants by low-energy infra-red radiation. An operative system was devised which has been in use for more than 4 years, with results which will be the subject of a subsequent report. "Moribund" and "healthy" simulated infants were warmed by conventional hot air blowers and by low-energy infra-red radiation from a conductive glass plate. The warming curves are presented.