<p>Reaction time (RT) tasks are used extensively to evaluate sensory-motor function for basic research and various clinical applications, so it is critical to identify methodological and environmental factors influencing RT. Further, changes in RT under different conditions may reveal novel aspects of sensory-motor processing. Previous studies have reported that RTs on Go/NoGo tasks can be modulated by changes in Go/NoGo stimulus ratio, potentially by influencing prestimulus preparatory activity and (or) poststimulus cognitive processing. To assess the underlying mechanisms, we measured RT in an S1–S2 version of the Go/NoGo task in which a warning stimulus (S1) preceded Go and NoGo stimuli (S2) delivered at two ratios (Go and NoGo frequencies), NoGo-Rare (25% NoGo/75% Go) and NoGo-Frequent (75% NoGo/25% Go). Simultaneous electroencephalographic recordings were acquired of the contingent negative variation (CNV) as a preparatory activity marker and both NoGoN2 and NoGoP3 as cognitive processing markers. Mean RT was shorter during the NoGo-Rare condition compared to the NoGo-Frequent condition, but there were no significant differences in CNV amplitudes between conditions. Alternatively, significant differences were observed in NoGoN2 amplitude, NoGoN2 latency, and NoGoP3 amplitude between conditions, suggesting that Go/NoGo stimulus ratio alters RT primarily by influencing poststimulus cognitive processing.</p>