In reaction time (RT) research on nonspecific preparation, the preparation period is often identified with the foreperiod (FP), the interval between the offset of a neutral warning stimulus (S1) and the onset of the reaction stimulus (S2). However, the "effective preparation period" may be longer than FP: nonspecific preparation may start prior to FP (e.g., at the onset of S1) and/or continue after it (i.e., in parallel with the reaction process). In four experiments, we factorially varied FP and an additional factor (S1-duration; S2-luminance; stimulus-response compatibility) that probed the effective preparation period outside the bounds of FP. By examining how equivalent RT-FP functions obtain at the different levels of the additional factor, we showed that nonspecific preparation (1) starts at the onset of S1 for brief FPs but at its offset for longer FPs and (2) continues in parallel with S2-encoding but stops prior to response selection. © 2007 Elsevier Inc. All rights reserved.