Current studies in transplantation of canine and human hearts have made it advisable to develop a method for detecting postmortem changes in hearts quickly and accurately. We use reversibility of the thermal responses of the viscoelastic properties of cardiac muscle to determine whether or not an excised heart is in a steady state. The test samples are taken from the tip of the atrial appendage and a report of cardiac muscle status may be made within 15 minutes, or before chemical signs of rigor mortis appear in the sampled organ. Methods Used in Designing the Test.— Rectangular segments of ventricles, atria, and atrial appendages were excised from in situ hearts of 17 dogs, 23 cats, and 13 rabbits anesthetized with 30 mg/kg of body weight of sodium pentobarbital. With this technique, a slit in the segment is slipped over two hooks to support it horizontally in a bath of known