The effects of thermal coagulation on the macroscopic optical transport parameters that govern the distribution of light in tissues were studied. The optical absorption coefficients, μa, and the reduced scattering coefficients, μs (1-g), were deduced from measurements of total transmission and total reflectance of HeNe laser radiation ($lamda = 633 and 594 nm) directed to thin slices of dog myocardium heated in vitro. The first optical changes were detected at 45° and, at temperatures above 65°, there was a 2-fold increase in absorption and a 7-fold increase in scattering. Transmission electron microscopy of laser-induced thermally coagulated lesions in rat myocardium (cw argon ion, λ = 514 nm) revealed ultrastructural alterations that were considered responsible for the increased scattering based on Mie theory. These microscopic alterations included disruption of mitochondria to form aggregates of electron dense granules and granular transformation of thermally coagulated proteins of the sarcomeres and cytoplasm. Our preliminary analyses suggest that the mitochondrial granules and the protein granules contribute to the increased scattering of light in thermally coagulated myocardium.