Two experiments examined hypotheses about the roles of probabilistic uncertainty and rhythmic context on attentional preparation as reflected by choice response times (RTs) to the final tone of auditory sequences. Nonisochronous sequences with tone timings either arranged metrically or scrambled were linked with one of three different sequence-final time intervals, or foreperiods (FPs), which varied randomly from trial to trial. Two primary results emerged. First, RTs were faster to target tones ending metrical rhythms than to targets ending scrambled rhythms. Second, metrical contexts elicited RTs that increased with FP duration, whereas scrambled contexts elicited RTs that often decreased with FP duration, despite equivalent variability of time intervals in metrical and scrambled contexts. The results suggest that time relations implied by metrical rhythms systematically modulate preparatory responses to sequence-final FPs.