The nature of processing demands during a letter-match task was investigated in an extension of the Posner and Boies (1971) paradigm. In Experiment I, a visual probe was employed in addition to an auditory probe in two different experimental conditions. The shape of the auditory probe reaction time (RT) function was similar to that found by Posner and Boies. However, in contrast to their findings, RT was greatly increased shortly after presentation of the first letter for the visual probe function. It was concluded that perceptual as well as postperceptual limitations on processing capacity exist. A second experiment provided further support for this hypothesis.