Several studies have shown that the amplitude of the Impedance CardioGraphy (ICG) signal can not simply be linked to cardiac stroke volumes. The time relationships of the ICG-signal, however, may provide vital clinical information, especially when the ICG is compared to the waves in the Electro-CardioGram (ECG). The time difference between the two signals reflects the time difference between the electrical and mechanical activity of the heart, which is controlled by the autonomic nervous system. The autonomic system is affected in several diseases, e.g. diabetes mellitus and Parkinson's disease (PD). Earlier studies have shown that a specific time interval, the Pre-Ejection Period (PEP), is related to sympathetic activation. The PEP, however, is based upon marker points that are often difficult to trace. The present study introduces a novel time interval, the Initial Systolic Time Interval (ISTI), defined as the time difference between the R-point in the ECG and the C-point in the ICG. The relationship between PEP and ISTI was investigated in three groups of subjects. The clinical significance of ISTI was investigated in two groups of PD patients and healthy controls. RR-interval variability (RRIV) and ISTI-variability (ISTIV) were determined at rest and during a hyperventilation stimulus. RRIV reflects parasympathetic activity. The ISTIV is considered to reflect sympathetic autonomic activity. During hyperventilation, the RRIV was significantly higher in controls than in PD patients. This suggests that PD patients have reduced parasympathetic responsiveness. At rest, ISTIV was significantly higher in patients than in controls, suggesting continuously varying levels of sympathetic activity to compensate for reduced parasympathetic function. In controls, ISTIV increased during hyperventilation. In PD patients, however, ISTIV did not increase during hyperventilation, probably because ISTIV in patients is already maximal at rest. It is concluded that ISTI may be a useful tool in evaluating autonomic regulation of cardiac function.