The purpose of the study was to reconcile the contradictory evidence reporting either a shortening or lengthening of the simple reaction time (RT) as a function of the intensity of a warning signal. The lengthening of auditory RT is obtained when short visual or auditory warning signals are of constant intensity within a session and succeeded by a fore‐period. Visual RT did not vary under similar circumstances. There was no effect when a warning signal of constant intensity overlapped in time with the response stimulus. The shortening of RT occurred with auditory warning signals of variable intensity and overlapping in time with the response stimulus. The shortening of RT was interpreted as facilitation‐by‐arousal. The lengthening of RT is a contextual effect but the data do not support accounts based on sensory‐neural adaptation or criterion