In this thesis, the effect of different pharmacological tools on heart rate variability is described. Heart rate varies with respiration, blood pressure, emotion, and other factors, and heart rate variability (HRV) is now one of the best predictors of fatal issues in cardiac failure and after myocardial infarction. HRV is influenced by a variety of reflexes. Furthermore, parallel studies of HRV and the myocardial adrenergic and muscarinic transduction system in experimental models of cardiac hypertrophy (CH) have suggested that the sinus-node myocardial phenotype may also play a role. Noninvasive techniques such as determining the standard error of the mean, time-domain analysis, and Fourier transformation are used to investigate HRV.