Background & Rationale: Heart rate variability analysis has provided scholars and clinicians with a powerful non-invasive tool for the assessment of cardiac autonomic function in health and disease. However, the interpretation of the information provided by this technique would be greatly facilitated by a more precise definition of 'normality'. The purpose of this investigation was to examine the alterations in cardiac autonomic function across a broad spectrum of ages in healthy males and females. Methods: Heart rate variability data during 20 min supine rest and orthostatic stress (10 min free standing) as well as 24-hour ambulatory Holter ECG recordings were obtained on 123 healthy volunteers (72 female/51 male). Subjects were arbitrarily classified into five categories: pediatric (PED; 5-12 yrs, n=22, 12 female:10 male), adolescent (ADO; 13-17 yrs, n=21, 13 female:8 male), adult (ADU; 18-30 yrs, n=26, 13 female:13 male), middle aged (MDA; 31-60 yrs, n=24, 15 female:9 male) and elderly (ELD; 61+ yrs, n=30, 19 female:11 male) age groups. Power spectral analysis (autoregressive) was determined from supine and standing acute data sets as well as six evenly spaced one hour periods during the Holter recording. Time domain variables (pNN50, R-MSSD, SDNN, SDANN & SDNN index) were derived from the 24-hour data sets. Results: Heart rate in the supine position declined progressively from age 5 years to 30 years but showed no further changes thereafter. In contrast, power spectral measures remained relatively stable in the younger age groups but subsequently exhibited a significant shift toward a higher LF:HF area signifying sympathetic dominance (or vagal withdrawal) in the MDA and ELD subjects. The heart rate and spectral response to orthostasis was most dramatic in the ADO subjects and exhibited a progressive decline in the three older age groups. With respect to the time domain variables, those parameters characterizing short term variability (pNN5O & R-MSSD) were stable in the PED, ADO and ADU subjects but significantly diminished in the two older age groups. In contrast, time domain variables encompassing long term (SDNN & SDANN) and intermediate (SDNN index) oscillations exhibited age-related increases reaching peak values in the ADU subjects and declining progressively thereafter. Power spectral analysis of the six evenly spaced one hour periods of the 24-hour holter recording revealed a diminished circadian rhythm for the majority of the frequency domain indices in the two oldest age groups. Conclusions: The present investigation revealed substantial evidence supporting the existence of an age dependent change in cardiac autonomic function. However, this process appeared to act homogeneously across gender. The similarity of these age dependent changes to those previously observed in pathological conditions commonly associated with autonomic neuropathy serves to emphasize the importance of HRV research aimed at the establishment of reference standards in healthy populations and a more precise definition of 'normal' autonomic neurocardiac function. / Thesis / Master of Science (MSc)