We studied the neural determinants of the second (i.e., high frequency, HF)- and third-order (i.e., low frequency, LF) spontaneous oscillations of heart period (R-R interval) and arterial pressure (AP) in conscious dogs, with the hypothesis that they might furnish quantitative markers of autonomic controlling activities. Spectral analysis of simultaneous R-R and AP variabilities quantified these oscillations that were also evaluated in units normalized by total power to focus on the balance of these two major components. At rest we observed a prevalent HF component (approximately 0.25 Hz) in R-R and AP variabilities that was synchronous with respiration. This HF component of R-R variability disappeared after atropine infusion and can be considered a marker mostly of vagal activity. When baroreceptor unloading, obtained by moderate hypotension, increased sympathetic activity the LF component increased in R-R, systolic, and diastolic AP variabilities. This increase in LF was not present after ganglionic blockade or after chronic arterial baroreceptor denervation. After chronic bilateral stellectomy, hypotension was not accompanied by an increase in LF component of R-R variability, while LF component remained in AP variability. An increase in LF component of R-R and AP variabilities was observed during transient coronary artery occlusion.