Observers detected letters in streams of digits under conditions of divided or selective attention for a period of 10 hr. There were marked practice effects both on the detectability of targets and on the response criteria used by observers. The detectability of targets and the response criteria were both strongly dependent on events in the contralateral channel. The same effects were observed in a second experiment in which the task was to detect animal names in streams of nouns. The data are similar to those obtained in recent experiments using pure tones, so that a unified theory of attention should be possible, valid alike for semantic and nonsemantic messages. Such a theory is outline.