On the basis of models describing optical and thermal transfer in tissue, it has been shown that unfocused, pulsed 577-nm laser irradiation of human skin selectively damages microvessels with little or no direct damage to other tissue structures such as the epidermis. This is in sharp contrast to the histologically nonselective necrosis induced by continuous lasers, in which essentially all structures near the site of exposure are damaged. We report here preliminary data in an animal model regarding mechanisms by which tunable dye lasers selectively alter blood and microvasculature. Irradiations of the hamster cheek pouch were used for direct microscopic observations of immediate changes in vessel integrity and morphology, and in the appearance and flow of blood. In 20- to 100-micron-diameter vessels, a sequence of brown discoloration and viscidification of blood, transient hemostasis, permanent hemostasis, and hemorrhage were observed with increasing exposure doses.