Cardiovascular health is negatively affected by overactivity of the sympathetic nervous system (SNS) during rest. Heart rate variability (HRV) has been used to predict SNS activity. The study investigated effects of a placebo, caffeine, and preworkout supplement (double blinded) on short term HRV before and after an acute bout of resistance exercise. Twelve subjects completed a trial with each supplement. Caffeine and exercise showed a significant decrease for Low Frequency (LF) Power normalized units (n.u.) (p=0.005) and a significant increase for High Frequency (HF) Power n.u. (p=0.010) immediately post exercise compared to exercise with placebo. Known effects that the combination of caffeine and exercise have on SNS activity do not agree with results found for HF Power n.u. and LF Power n.u using traditional interpretation of these indices for SNS activity. This suggests that the relationship between the cardiovascular autonomic control system and HRV is more complex than previously thought.