Recent work with humans and animals has suggested that the cholinergic system plays an important role in the active control of attention. This study was designed to investigate the effects of scopolamine upon subjects' ability to utilize knowledge of the spatial probability bias in a display in the detection of briefly-presented target letters. Results showed a significant interaction between drug condition (scopolamine 0.9 mg versus Placebo) and target location probability, which indicated that detection of targets in high probability locations decreased under the drug, while detection in low probability locations increased. These results offer support for the notion that scopolamine reduces the efficiency of information encoding because it impairs the optimal utilization of attentional resources.