Objective: The contribution of movement-related potentials (MRPs) to the Go/NoGo N2 and P3 ‘inhibitory’ effects is controversial. This study examined these components in overt and covert response inhibition tasks. Methods: Twenty adult participants counted or button-pressed in response to frequent (60%) and rare (20%) Go stimuli in a Go/NoGo task with equiprobable rare (20%) NoGo stimuli. Results: The N2 NoGo effect did not differ between Count and Press responses, but the P3 NoGo effect was amplified during the Press task. Additionally, subtraction of the ERP waveform for Count NoGo from Press NoGo trials revealed a positivity between 200 and 400 ms, occurring maximally over the central region, contralateral to the responding hand. This difference wave became significant at 210–260 ms, close to the estimated time taken to stop an overt response. Conclusions: The N2 NoGo effect may reflect a non-motoric stage of inhibition, or recognition of the need for inhibition, while the NoGo P3 may overlap with a positive MRP occurring specifically on trials where overt motor responses must be inhibited. Significance: The study confirms that the N2 and P3 NoGo effects are not solely due to movement-related potentials, and posits the NoGo P3 as a marker of motor inhibition.