Changes in the function of the autonomic nervous system underlying changes in heart rate variability are not fully understood. Furthermore, decreased heart rate variability has been found to be related to poor prognosis, for example, in patients with coronary artery disease. Our aim was to study how modulation in sympathetic stimulation at various frequencies is transferred into heart rate variation, and how the interaction between sympathetic and parasympathetic inputs can affect the high-frequency component of heart rate variability. We stimulated electrically cardiac sympathetic and vagal nerves in anaesthetized, vagotomized, spinal anaesthetized dogs. We controlled the frequency and magnitude of the modulation in programmed stimulation patterns and analysed the resulting changes in heart rate variability by power spectral analysis. We found that modulations in sympathetic stimulation were reflected in the high-frequency component of heart rate variability, as well as in the low- and medium-frequency components. In addition, a novel finding was that sympathetic stimulation reduced the magnitude of the high-frequency variations caused by vagal stimulation. This suggests that, although the high-frequency component of heart rate variability is mainly under parasympathetic regulation, it may also be influenced by the sympathetic nervous system.