The effect of methylmercury hydroxide (MMH) on the frequency of spontaneously beating atrial-SA node and on the isometric tension of electrically stimulated left atrial (1 Hz) preparation isolated from rats were investigated. The tissues were also fixed for ultrastructural study at the end of the experiments. We found that MMH, at low doses (0.5 and 2ppm), increased the frequency of contractions and the isometric tension of isolated atria and decreased the isometric tension of isolated papillar muscles, and at higher doses (greater than 2ppm), decreased all the above parameter. Morphologically, there were dilation and swelling of mitochondrial and sarcoplasmic reticulum. Occasionally, deposits of amorphous material in the mitochondria and myofibrillary degeneration of myocardial fibers were noted. It is concluded the MMH has direct effect on intracellular organelles (mitochondria, sarcoplasmic reticulum, and myofibril) of myocardial tissue and induces functional changes of atria and papillary muscles