Fresh myocardial segments from fetal and adult sheep, and from newborn and adult pigs were exposed to continuous mode argon laser irradiation in saline medium. Additionally, myocardial segments from newborn pigs were exposed to laser irradiation in fresh, heparin-ized blood medium. The irradiation distance from the tip of the quartz fiber to the tissue varied between contact and 20 mm, and power output at the fiber tip varied between 1 and 8 watts. Exposure time was kept constant at 2 seconds. Tissue debris was also processed for study by scanning and transmission electron microscopy. There was no difference in myocardial tissue response between sheep and pigs, nor was there a difference in response between young and adult animals. In both saline and blood media, there was a sharp decrease in burn depth with increasing irradiation distance. With increasing irradiation distances in saline medium, burn diameter increased initially and then plateaued; while with increasing irradiation distance in blood medium, the burn diameter decreased sharply. When the fiber tip was in contact with the tissue, the diameter of burn was greater in blood than saline, while the depth of burn was similar. Filtration of the tissue bath demonstrated particles as large as 3 mm in length which were composed of deformed and coagulated whole tissue segments. Electron and scanning micrography of the bath media identified intracellular components and fragments of burst cells. In conclusion, we have found no difference in adult vs. newborn, or sheep vs. porcine myocardial response to fiberoptic argon laser irradiation. The most critical factors affecting width and depth of burn were the distance of the fiber tip from the tissue, and the medium in which the tissue was bathed. Of particular clinical importance was the fact that the burn width and depth drastically decreased when blood was present between the laser fiber and the tissue.