Much of the fluctuation in choice RT data is assumed to arise from variations in perceptual and response biases correlated with sequential patterns of trial events. Inserting a predictive cue prior to the stimulus on each trial apparently induces a strong bias not only toward that particular stimulus, but also toward its modality and toward the associated response. Trial-to-trial sequential effects under cued conditions were markedly smaller than those obtained in a noncued control condition. © 1970 Psychonomic Society, Inc.