is known that microwaves can damage tissue but the cause is poorly understood, e.g., can tissue be damaged in the absence of undesirable heat levels? To answer this question 106 isolated 72 hour embryonic chick hearts were subjected to radar waves at a frequency of 24,000 megacycles, a wave length of 1.25 cm and exposures of 478, 297, 167 and 74 mw/cm2 for lengths of time varying between a few seconds and three minutes. Electrocardiograms were taken as a continuous run before, during and after radar treatment. The hearts were planted in blood plasma on the surface of saran sheet and microwaves were directed through the sheet at the heart. Where necessary a stream of cold air was used to offset undesirable elevation of heat. At exposures of 478 and 297 mw/cm2 damage was swift and devastating. No conclusion seemed justified. However, at an exposure of 167 mw/cm2 it was possible to prevent elevation of temperatures above 38°C. Significantly the hearts showed damage in the ECG, i.e., shortening of the QT interval, increased height and breadth of the T wave and the appearance of a large U wave.Finally at 74 mw/cm2 it was possible to control heat merely by maintaining an ambient temperature below 25°C. Of twenty-five hearts so exposed, all showed ECG changes mentioned in the previous paragraph. The conclusion seems justified, that radar waves can damaee tissue at exposures which do not raise the temperature to a -harmful level.