The purpose of this investigation was to determine if 8 exercise endurance training (ET) sessions over 2 weeks significantly alters cardiac autonomic modulation, as measured by heart rate variability (HRV). Twenty-four college-aged males were recruited for this study and were randomized into either an exercise group (EX; n=12) or a control group (CT; n=12). EX underwent 2 weeks of ET on a cycle-ergometer (frequency: four times/week; duration: 40 minutes; intensity 80-85% HR reserve), whereas CT was instructed not to alter their previous level of physical activity. Five-minute ECG tracings were collected for HRV under the following conditions: 5 minutes of paced breathing at a frequency of 12 breaths/minute (PB), 5 minutes of spontaneous breathing (SB1), 5 minutes of 70-degree head-up tilt on a tilt table (TILT), and a second 5-minute period of spontaneous breathing (SB2). The data were collected on 5 occasions (Test 1-Test 5) during a 2-week period. HRV was reported as the standard deviation of RR intervals, and as normalized units (NU) of the natural logarithm of high-and low-frequency power (lnHF and lnLF). A mixed-model analysis of variance (ANOVA) with repeated measures was used to evaluate any Group X Time interaction, or main effects of Group or Time on HRV. Alpha was set at 0.05. EX exhibited a significant increase in peak oxygen consumption (8%), whereas this parameter was unchanged in CT. During PB, ANOVA revealed a Group X Time interaction such that EX exhibited decreases in lnLFNU (2.89 vs. 3.45 ln%) and lnLF/lnHF (0.82 vs. 0.90) during Test 5 compared to Test 1, while there were no such differences across time in CT. Additionally, there was a Group X Time interaction during TILT such that EX exhibited lower lnLFNU (4.12 vs. 4.42 ln%) and lnLF/lnHF (1.12 vs. 1.46), and greater lnHFNU (3.52 vs. 2.53 ln%) during Test 5 compared to Test 1, while these indices were unchanged across time in CT. These data suggest that 8 ET vii sessions over 2 weeks increases vagal modulation of the heart. Furthermore, these data suggest that it takes at least 8 ET sessions to induce such changes.