During tasks requiring response inhibition, intra-individual response time variability, a measure of motor response preparation, has been found to correlate with errors of commission, such that individuals with higher variability show increased commission errors. This study used fMRI to examine the neural correlates of response variability in 30 typically developing children, ages 8–12, using a simplified Go/No-go task with minimal cognitive demands. Lower variability was associated with Go activation in the anterior cerebellum (culmen) and with No-go activation in the rostral supplementary motor area (pre-SMA), the postcentral gyrus, the anterior cerebellum (culmen) and the inferior parietal lobule. For both Go and No-go events, higher variability was associated with activation in prefrontal cortex and the caudate. The findings have implications for neuropsychiatric disorders such as ADHD and suggest that during response inhibition, children with more consistent performance are able to rely on premotor circuits involving the pre-SMA, important for response selection; those with less consistent performance instead recruit prefrontal circuits involved in more complex aspects of behavioral control.