Fiberoptic-coupled, Neodymium-Yag laser beams were directed at induced bleeding mucosal lesions in the stomachs of 13 dogs. Hemostasis was achieved in all cases. The lesions induced by photocoagulation were examined by light microscopy and transmission electron microscopy. Phototherapy induces an endothelial reaction followed by the formation of a platelet thrombus in the damaged vessels. Hemostasis starts shortly after coagulation therapy has been performed.