For decades the allometric relationship between body weight (BW) and heart rate (HR) was described as inversely proportional between the various domestic species, but currently has been refuted. It is more likely this relationship is reflexive of temperament and sympathetic autonomic stimulation of the sinus node in small-breed dogs compared to large breeds. One hundred fifteen tracings correlating age and body weight (< 5 kg; 5–10 kg; 10–25 kg; 25–45 kg) were analyzed with HR (Pearson) and comparing the HR according to logarithmic equation HR = 241 x BW - 0,25 with HR electrocardiographic (Tukey and Kruskal-Wallis p < 0.05). Dogs weighing under 5 pounds and between 25 to 45 kg, according to the HR logarithmic equation was significantly different from HR electrocardiographic (p < 0.001), no correlation between HR and body weight. In dogs 5 to 10 kg there was no statistical difference between the HRs, however no correlation HR/BW and negative correlation HR/age. In 10–25 kg, no significant difference between the HR and correlated negatively HR/BW. It was noticed that no allometric relationship HR/BW in dogs under 5 kg and between 25 to 45 kg, but the relationship can be seen in dogs weighing 5–25 kg. It was concluded that the relationship between HR and BW occurs only in certain ranges of body weight, and that breeds in small (< 5 kg) and the large (25–45 kg) the autonomic tone and temperament of the animal influence the HR.