Laser beam constitutes a potential therapy in modern cardiology. Although its myocardial effects are known, they have been insufficiently evaluated at the surviving myocytes zone surrounding the evaporated crater. In order to assess myocyte cellular and organelar damage, we studied 8 isolated canine hearts radiated with different intensities by a CO2He laser beam 2.4, 3.5, 5, 15, 25, 30 and 40 Watts, varying the exposition time from 1 to 6 sec. Color photographs from the post-radiated lesions on front and depth were obtained. Their volumes were measured applying the cylinder and the cut out-conus formulas according to the shape of the lesions and then the involved tissues were embedded in paraffin for histological studies. Previously, samples of the lesions were fixed in glutaraldehyde for ultrastructural studies. Volumes of the craters were from 0.0004 to 19.57 mm3. Three layers were observed: a) a carbonized internal lining which measured x 15 microns in thickness; b) a coagulation necrotic zone ("gruyere cheese-like"), 70 microns thickness; c) finally more peripherally, a layer consisting of myocytes with patchy homogeneous cytoplasm, and scare positivity for the Barbeito López Trichome Stain (a positive technique in cases of myocardial damage). Ultrastructurally, crater peripherical cells showed cytosol and mitochondrial edema without membrane disruptions. These findings suggest that myocytes surrounding the laser beam crater, show reversible lesions. Therefore laser beam apperrs a safe procedure to be used in myocardium.