Past, present, and future actions must be regulated online to produce sequences of actions, but the regulation process is not well understood because of measurement limitations. We provide the first direct tests of the parallel action regulation hypothesis during sequencing in humans. We used transcranial magnetic stimulation to probe the level of excitation for flexion of the right index finger during typing. Motor evoked potentials (MEPs) were recorded at the onset of typing 5-letter words and nonwords. A single letter typed by the right index finger varied across letter positions 1 to 5. MEP amplitude was largest for the upcoming action in the second position and decreased monotonically across future serial positions, suggesting a serial inhibition process regulates all future actions in parallel during sequencing. This is the most direct human evidence to date corroborating models of sequence production that assume parallel regulation of actions.