Human observers fixated the center of a search array and were required to discriminate the color of an odd target if it was present. The array consisted of horizontal or vertical black or white bars. In the simple case, only orientation was necessary to define the odd target, whereas in the conjunctive case, both orientation and color were necessary. A cue located at the critical target position was either visible all the time (sustained cuing) or it appeared at a short variable delay before the array presentation (transient cuing). Sustained visual cuing enhanced perception greatly in the conjunctive, but not in the simple condition. Perception of the odd target in the conjunctive display was improved even further by transient cuing, and peak discrimination performance occurred if the cue preceded the target array by 70-150 msec. Longer delays led to a marked downturn in performance. Control experiments indicated that this transient attentional component was independent of the observers' prior knowledge of target position and was not subject to voluntary control. We provide evidence to suggest that the transient component does not originate at the earliest stages of visual processing, since it could not be extended in duration by flickering the cue, nor did it require a local sensory transient to trigger its onset. Neither the variation in retinal eccentricity nor changing the paradigm to a vernier acuity task altered the basic pattern of results. Our findings indicate the existence of a sustained and a transient component of attention, and we hypothesize that of the two, the transient component is operative at an earlier stage of visual cortical processing.