This study investigates the beat-to-beat oscillations in heart rate and arterial pressure under various conditions to assess the interaction between sympathetic and parasympathetic regulatory activities. The analysis was performed on 57 healthy subjects and 12 conscious dogs. In humans, the R-R interval variability was analyzed using spectral techniques, revealing two major components: a high-frequency (HF) component at ~0.25 Hz and a low-frequency (LF) component at ~0.1 Hz. During tilt, the LF component became predominant, and acute and chronic β-adrenergic receptor blockade altered the LF and HF components. Controlled respiration increased the HF component and reduced the LF component. In conscious dogs, sympathetic activity was assessed by measuring R-R interval and arterial pressure variabilities before and after bilateral stellectomy. Sympathetic activity was associated with an increase in LF components, which were absent or small under control conditions. Bilateral stellectomy prevented this increase, suggesting that sympathetic nerves are crucial for low-frequency oscillations in R-R interval. The study concludes that spectral analysis of R-R interval and arterial pressure variabilities can provide valuable insights into the balance between sympathetic and vagal activities, which may have implications for understanding physiological and pathological conditions.This study investigates the beat-to-beat oscillations in heart rate and arterial pressure under various conditions to assess the interaction between sympathetic and parasympathetic regulatory activities. The analysis was performed on 57 healthy subjects and 12 conscious dogs. In humans, the R-R interval variability was analyzed using spectral techniques, revealing two major components: a high-frequency (HF) component at ~0.25 Hz and a low-frequency (LF) component at ~0.1 Hz. During tilt, the LF component became predominant, and acute and chronic β-adrenergic receptor blockade altered the LF and HF components. Controlled respiration increased the HF component and reduced the LF component. In conscious dogs, sympathetic activity was assessed by measuring R-R interval and arterial pressure variabilities before and after bilateral stellectomy. Sympathetic activity was associated with an increase in LF components, which were absent or small under control conditions. Bilateral stellectomy prevented this increase, suggesting that sympathetic nerves are crucial for low-frequency oscillations in R-R interval. The study concludes that spectral analysis of R-R interval and arterial pressure variabilities can provide valuable insights into the balance between sympathetic and vagal activities, which may have implications for understanding physiological and pathological conditions.