The electrocardiogram (ECGorEKG) is a diagnosis toolthat measures and records the electrical activityof the heart in exquisite detail. Itis a non-invasive method which can offer information about the cardiac activity, for a huge number of species, in one hand having an interspecific comparative image and in the other hand, allowing to track down the pathological changes of the cardiac activity. Even that the cardiac aspects in fish (Sander lucioperca), constrictor snake (Boa constrictor) and dog are different from the morphological point of view. By comparing the three obtained records,we could demonstrate that there are identical aspects of the wave morphology, segments and intervals. The two chambers heart(Sander lucioperca), the three chambersheart (Sanderlucioperca)and the four chambersheart in dogs (Canis familiaris),the excitatory and conductive systems are identical, being made by the sinusnodeor sinoatrial node(S-A),the atrioventricular node (A-V),Hissbundle and Purkinjefibers.