The purpose of this study was to examine the relationship of physical activity and cognitive function (as determined by reaction time and the trail-making test) in active versus non-active participants. Participants were divided into one of four groups: active experimental, active control, non-active experimental and non-active control. All groups completed a complex cognitive task (the trail-making test) as well as a set of reaction time tasks both before and after the experimental session. The experimental groups completed a 30-min exercise session while the control groups monitored the physical activity of the experimental group. In addition to the measures of cognitive function, heart rate variability was recorded during the pre- and post-tests. There was significant cognitive performance improvement in tasks with a higher cognitive and perceptual component. Heart rate variability data indicated that a moderate level of arousal based on sympathetic nervous system activity post exercise was associated with an increase in cognitive performance. The findings are discussed in light of the inverted-U hypothesis.