Transient spatial attention has been shown to alter different stimulus properties, such as perceived contrast and speed of motion. Interestingly, recent studies seem to suggest that involuntary shifts of spatial attention can also occur on the basis of the contents of Working Memory (WM). According to the biased competition model of visual attention, stimuli in the visual field that match those currently maintained in working memory receive the highest attentional priority. In a series of five experiments we addressed whether WM-based attention shifts alter perception, and specifically speed of motion. We found that response times for target discrimination were shorter at the location where an object matching the one held in WM was presented, thus confirming WM-based attention shifts. In contrast, results from other experiments, in which a speed-discrimination task was used, failed to reveal any modulation of motion speed perception by the content of WM. At present these data suggest that WM-based attention operates at different (later) stages of visual analysis than transient attention.