RTs were measured in a two-alternative task. 8 to 10 undergraduate students served as subjects in each series of experiment. The subject was asked to assign an expectancy rating on an 11-point scale before each stimulus presentation. In Experiment I, the stimulus presentation probabilities were biased (0.75: 0.25 or 0.25: 0.75), but they were independent of the preceding stimulus. In Experiment II, the presentation probabilities of two stimuli were equal, but the same stimulus was repeated with a conditional probability of 0.75. The RT to a stimulus became shorter as the expectancy to that stimulus became stronger when the expectancy was directed to the correct stimulus. But it was nearly constant when the expectancy was directed to the incorrect stimulus. Probability effect and repetition effect were found in Experiments I and II, respectively. It was concluded that shorter RTs to the more probable stimulus and to the repeated stimulus can be attributed mostly to stronger expectancies of these stimuli.