We measured manual reaction time in normal human subjects to confirm that an eccentric visual signal has a biphasic effect on covert attention and eye movements. First, it summons attention and biases a saccade toward the signal; a subsequent inhibition of return then slows responses to signals at that location. A temporal hemifield dominance for inhibition of return was shown; this finding coverges with observations in neurologic patients to suggest that it is mediated by midbrain pathways. Endogenous orienting of attention, from a central arrow cue, did not activate inhibition of return, whereas endogenous saccade preparation did so as effectively as an exogenous signal, even when no saccade was made. Inhibition of return is activated by midbrain oculomotor pathways and may function as a location "tagging" mechanism to optimize efficiency of visual search. The selectivity of visual perception is guided by brain mechanisms that orient attention in the visual field. One manifestation of visual orienting is overt, consisting of move- ments of the head or eyes, or both, to focus on the attended spatial position. Covert orienting may occur, however, inde- pendently of motor activity and serves to align attention to a spatial location to enhance the processing of or response to signals that occur there. Researchers have studied covert orienting in humans by measuring the pattern of facilitation and inhibition in responding to an eccentric visual target, which result from preliminary cues that either correctly pre- pare the subject to detect the target at the cued location or summon attention elsewhere in the visual field (Posner, 1980). In this kind of experiment, the eyes remain fixed in the center of the display, and inasmuch the required response is always a simple reaction time (RT) manual key press regardless of target location, these facilitations and inhibitions in RT per- formance serve as a measure of visual attention that is inde- pendent of eye position or response selection. This method has also been used in experiments in which eye movements are made to show that when a peripheral signal is the target of a saccade, attention moves to the signal before the onset of the saccade (Posner & Cohen, 1980) and that under some circumstances, attention can move in one direction while the eyes move in another (Posner, 1980). A major goal of research in visual attention, addressed in this study, is to determine how neural systems responsible for the covert allocation of visual attention are integrated with