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( )IR EXPERIMENTS were designed to determine the effects of focused and unfocused laser radiation upon the heads of animals. This report is the result of our first set of experiments. While preparing these experiments, Drs. Sam Fine and Edmund Klein told us of their unreported experiments in which they produced subarachnoid hemorrhage and death in mice within a few minutes by directing an unfocused l)eam from a rubv rod laser onto their heads. They did not do histologic studies at that time. However, unlike the experiments of Fine and Klein, we decided not to shave the heads of most of our animals. This point undoubtedly explains the differences in some of our observations. Our first set of experiments was designed to repeat those of Fine and Klein and extend them by determining the effect of pulsed laser beams directed in various ways: converging onto the brain through the intact skull, diverging into the brain, converging on the other side of the head, and unfocused. A good review of the subject of optical masers (lasers) in biology and medicine is found in the articles by Litwin and (Jlew (i), Malt and Townes (3), and in other presentations in this symposium. Therefore, we will not review the basic theory and current application of lasers, but will confine ourselves to the specific experiments we did and the conclusions reached. The lesions of the skin were studied by Dr. E. B. Helwig, Chief of the DermatoU ^ic Pathology Branch of the Armed Forces Institute of Pathology, and his findings will be the subject of a separate report on the effects of laser radiation on the skin. We shall descrÜMonly the gross appearance of the skin lesions here.