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Fresh isolated myocardial, elastic-arterial, and valvular tissues from seven canine hearts were irradiated by argon laser. Irradiation was transmitted through 300 and 400 micron flexible quartz fiberoptic elements. Minimal power densities for vaporization of the myocardial, arterial, and valvular tissues were 80, 90, and 110 W/cm2, respectively, with maximal vaporization distances (fiberoptic tip to tissue) of 4 mm, 1 mm, and 1 mm, respectively. Irradiation of the valves at power densities approaching perforation caused contraction of the tissue. When tissue vaporization occurred, histologic examination of irradiated tissues showed a central crater surrounded by sequential layers of char, vaporization, and coagulation necrosis. These findings were common to all cardiac tissues. Additional findings unique to myocardium were a normal-appearing myocardial layer (skip-area), circumferential halo, and tissue clefts. Elastic arteries showed concentration of necrosis around the collagen and elastic fibers. Valvular damage was the most extensive and also included contraction of adjacent valvular tissue and endocardial sloughing.