Low heart rate variability (HRV), indicative of reduced cardiac parasympathetic efferent activity, has been reported in obese individuals and is associated with increased cardiovascular risk [1]. Endurance exercise training (ET) has been shown to improve HRV in a range of populations. High intensity interval training (HIT, 9 minutes per week) can elicit remarkable metabolic adaptations and improvements in maximal aerobic capacity (VO2max), equivalent to traditional moderate intensity ET (~5 hours per week) [2]. However, it is presently unclear whether HIT training and ET elicit comparable changes in HRV in obese individuals. To begin to examine this, 11 sedentary, obese, male subjects were assessed before and after either 6 weeks of ET (n=5, 27±2 years) or 4 weeks of HIT (n=6, 26±3 years). ET consisted of cycling at 65% of VO2peak for 40-60 minutes, 5 times a week. HIT consisted of 30 second intervals (4-7 repetitions) at constant load of 200% maximal aerobic power (Wmax) with 2 minutes recovery between intervals, 3 times a week. At baseline and post-training, subjects underwent a maximal incremental exercise test on an electrically braked cycle ergometer to determine VO2max, Wmax, maximal heart rate (HRmax), and heart rate recovery (HRrec) at 1 and 3 min post-exercise. HRV was determined on a separate day from 5 min electrocardiogram recordings obtained with subjects resting quietly in the supine position. HRV parameters were determined using time and frequency domain methods (Kubios HRV software). Statistical analysis was performed using two-way repeated measures ANOVA and post hoc Holm Sidak tests. At baseline, body mass index (BMI; 33±2 vs. 35±1 kg.m2) and % body fat (31±1 vs. 31±3 %) were similar in the ET and HIT groups (P>0.05). % body fat decreased with training similarly in both groups (by ~1%, P<0.05), while BMI was unchanged. VO2max and Wmax were similar in ET and HIT groups at baseline (P>0.05) and were increased similarly post-training (VO2max; ET, 36±2 to 40±2; HIT, 37±3 to 39±2 ml.kg.min-1; baseline vs. post training P<0.01). HRmax was slightly, but significantly reduced with exercise training (ET, ~2 bpm; HIT, ~6 bpm, baseline vs. post-training P<0.05). HRrec and HRV were not different between groups, and not significantly altered following either ET or HIT training (P>0.05). Our preliminary findings indicate that 6 weeks of HIT or ET in obese individuals elicits similar increases in aerobic exercise capacity (VO2max) and reductions in % body fat, whereas HRV remains unchanged. Further studies are required to fully elucidate how exercise training modality may differentially affect HR regulation in obese individuals.