Binary choice-reaction times for 123 males, 25-79 years of age, were obtained in a sequence of discrete trials in which the proportion of occasions that the same stimulus was presented twice in succession varied from .25, .50, and .75 or from .75, .50, and .25 over successive thirds of the sequence. Stimulus alternations were responded to more rapidly than stimulus repetitions by subjects in all age groups, especially in trial blocks with .75 alterations. The absolute differences in mean reaction time to stimulus alterations and repetitions increased with age. The results are consistent with the hypothesis that reaction times are affected both by pre-existing expectations for stimulus alternations as well as expectations for stimulus alternations or repetitions developed during the course of the experiment. Mean response latency as well as between- and within-subject variability in performance increased with age. Analyses of frequency distributions of the reaction times indicated that the psychomotor component of reaction time increases with age while the decision-making component becomes more variable.