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Transcranial Doppler sonography*** a low-cost noninvasive procedure which allows continuous monitoring of blood flow in the left and right middle cerebral arteries, was employed while participants performed a 40-min vigil. Two levels of signal salience (high and low) were combined factorially with four conditions in which monitors were forewarned about the imminent arrival of critical signals (100%, 80%, and 40% reliable cueing and a no-cue control). For both levels of signal salience, the frequency of signal detections remained stable over time in the 100% cue-reliability condition but declined over time in the remaining cue conditions, so that by the end of the vigil performance efficiency was best in the 100% condition followed in order by the 80%, 40%, and no-cue conditions. These performance effects for cueing were mirroredby blood flow measurements taken from the right hemisphere in conjunction with low salience signals. No effects for cueing were observable in blood flow recordings taken from the left hemisphere in conjunction with either high or low salience signals. The results indicate that blood flow and signal detection in vigilance are at least partially related to common mechanisms, such as the expenditure of information-processing resources. They are also consistent with past findings indicating that vigilance performance is right-lateralized.