Abstract: Burn lesions were produced by radiant energy, 3100 A to 22000 A, on depilated rat skin. The gross and microscopic pathological changes so caused are described and correlated with the amounts of incident energy and the rate of energy delivery. Within the limits studied, 0.2 to 64 cal./cm.2/sec. and 0.1 to 8.0 seconds, it was found that increasing the rate of energy delivery lowered the amount of energy required to produce a specified degree of tissue injury. The tissue changes were in many respects similar to those described for contact burns of the skin of rats and other animals. The findings suggest the action of such intensities of this spectrum of radiant energy is essentially of a thermal nature.