There is little information on density of myocardial tissue; the value most commonly quoted is that of Creese (1) which was for the rat diaphragm. Herd et al. (2) provided a single value of 1.05 g/ml for the left ventricle of the dog; Allen, Krzywicki and Roberts (3) reported the density of fat-free myocardium of rats, guinea pigs or rabbits as 1.054 or 1.058 at 27 or 37°. In this laboratory, Richmond, La Force and Bassingthwaighte (4) found values in cats of 1.057 g/ml (SD = 0.006, N = 9) for left ventricle and 1.053 g/ml (SD = 0.002, N = 6) for right ventricle at 23°, with a mean water content of 0.796 g/g (SD = 0.016, N = 6) for right venticle. In view of the widespread use of the dog as an experimental animal, particularly for studies on coronary flow, it seemed pertinent to provide data on densities of myocardium from various locations in canine ventricles. Since densities depend on the content of solid, fat, and water, we also estimated water content on myocardial pieces adjacent to those whose density was measured. The appendix provides a general approach to the relationship between density and tissue composition. Materials and Methods. Hearts were excised quickly from dogs killed by pentobarbital at the end of experiments done by our colleagues wherein the cardiovascular system was not excessively disturbed, or after exsanguination under the anesthesia (when only the blood was used for other experiments). The hearts were not used if they showed macroscopic extravasation of blood or if the previous history of the dog suggested illness or dehydration. The density was measured by the copper sulfate method of Phillips et al. (5) at 22.5 to 23.5° using a series of solutions with densities ranging from 1.048 to 1.070 at increments of 0.001.