Recently the circadian rhythm of fluctuations in heart rate variability (HRV) has gained increasing attention, and the use of appropriate treatment corresponding to the circadian rhythm has become an important issue. However, the question has been raised as to whether the rhythm of fluctuation is only limited to HRV, and if a rhythm is also present in the contraction of the heart. The mathematical technique of electrical circuit simulation of the cardiovascular system was used to observe the Emax rhythm. In this study, the therapeutic effect of various drugs, including atrial natriuretic polypeptide (ANP), was confirmed. The effect of inotropic drugs including ANP on Emax was observed, and the results based on an old and a new method of determining Emax . were examined. Furthermore, the chaos analysis of Emax was also made. The time series of Emax showed an increased complexity due to the administration of ANP. In conclusion, it might be important to consider not only the fluctuation of HRV, but also an analysis of the fluctuation in the contraction of the heart.