Determination of the efficacy and safety of endoscopic laser photocoagulation to control upper gastrointestinal bleeding is prerequisite to the general application of this treatment in humans. We studied photocoagulation hemostasis, penetration, and perforation produced by a 55-W neodymium-yag (Nd:YAG) laser to control standardized experimental gastric bleeding lesions in a heparinized canine model. Photocoagulation of bleeding lesions was 100% successful with an application time of 3.56 +/- 1.65 sec (mean +/- SD). Histologic examintion of the gastric wall revealed a depth of injury to the muscularis externa of 40-100% with greater than 2 sec photocoagulation. Continuous photocoagulation exceeding 4 sec produced an 80-100% depth of muscle injury. Perforation of the gastric wall occurred after 9.6 +/- 1.5 sec, and all dogs studied after perforation survived. These studies indicate that Nd:YAG photocoagulation is an effective method to control experimental gastric bleeding lesions with a ratio between mean photocoagulation hemostasis and perforation times of approximatley 1:3. Further studies are required to define the implications of photocoagulation injury to muscle and the role of the gastric serosa in laser applications.