Three experiments investigated whether spatial and nonspatial components of visual attention would be influenced by changes in (healthy, young) subjects' level of alertness and whether such effects on separable components would occur independently of each other. The experiments used a no-cue/alerting-cue design with varying cue-target stimulus onset asynchronies in two different whole-report paradigms based on Bundesen's (1990) theory of visual attention, which permits spatial and nonspatial components of selective attention to be assessed independently. The results revealed the level of alertness to affect both the spatial distribution of attentional weighting and processing speed, but not visual short-term memory capacity, with the effect on processing speed preceding that on the spatial distribution of attentional weighting. This pattern indicates that the level of alertness influences both spatial and nonspatial component mechanisms of visual attention and that these two effects develop independently of each other; moreover, it suggests that intrinsic and phasic alertness effects involve the same processing route, on which spatial and nonspatial mechanisms are mediated by independent processing systems that are activated, due to increased alertness, in temporal succession. (Contains 4 footnotes, 1 table, and 15 figures.)