Abstract Body (Do not enter title and authors here): Metabolic Syndrome (MetS) affects ~35% of the adult population in the US and increases the risk for cardiovascular complications and all-cause mortality. Altered heart rate variability occurs with MetS, but factors underlying the process remain unclear. To gain information on effects of MetS on heart rhythm dynamics, a novel algorithm was developed to analyse the distribution of RR intervals of ECGs collected in male and female mice with MetS, which was induced by Western diet (WD). Animals on regular chow were used as control (Ctrl). Using an automated approach, RR intervals of ECG recordings were sorted in ascending order and normalized with respect to the total number of intervals, using a percentile function. For Ctrl and WD mice, sorted and normalized RR interval sequences (sortRR) were combined to generate, for each group, a series of median RR interval values for the entire RR interval range. SortRR sequences for Ctrl and WD mice were not different in the range 0-33%, corresponding to short RR interval durations but RR intervals were significantly shorter in WD mice, with respect to Ctrl, in the range 33-100%, corresponding to longer RR intervals. Thus, WD mice have limited capability to lower heart rate. Moreover, to quantify RR interval deviation from its median value, sortRR series were offset with respect to the median RR interval. The resulting sortΔRR sequences were aggregated for Ctrl and WD mice and statistically compared. ΔRR were significantly smaller in WD mice, with respect to Ctrl, for 93% of the sortΔRR sequences. Thus, WD mice have reduced RR interval span and reduced variability, with respect to Ctrl animals. Similar results were observed for the two sexes. In the presence of a β-blocker, RR intervals were comparable from 0-97% of the sortRR sequence in Ctrl and WD mice, whereas ΔRR were smaller in WD mice, with respect to Ctrl, for 97% of the sortΔRR series. In the presence of the parasympathetic inhibitor atropine, duration of RR intervals was shorter in WD with respect to Ctrl mice over the entire range of the sortRR. In contrast, sortΔRR were largely similar for the two groups. Thus, sympathetic tone inhibition abrogates differences in RR interval duration whereas parasympathetic tone inhibition abrogates difference in RR variability between Ctrl and WD mice. In conclusion, sympathoexcitation and parasympathetic withdrawal with MetS reduce heart rate reserve by affecting, respectively, RR interval duration and its variability.