Examined the effect of temporal uncertainty on the latency operating characteristic (LOC), defined as the trade-off between speed and accuracy, using 6 graduate and 6 undergraduate students. Foreperiods were either constant (1,000 msec), 1 of 2 equally likely but unnoticeably different durations (975 and 1,025 msec), or 1 of 2 equally likely and noticeably different durations (750 and 1,250 msec). Mean reaction time increased with temporal uncertainty, but this was not always so for the LOC. Most efficient performance was obtained when the foreperiod was 975 msec, and poorest performance occurred in the 1,250-msec condition. Better performance with the early stimulus (at 975 msec) is contrary to anticipation and preparatory set models for the effects of temporal uncertainty but is consistent with a diluted sample model in which temporal uncertainty affects the temporal integration of sensory data. (PsycINFO Database Record (c) 2006 APA, all rights reserved)