The effects of 192 IgG-saporin-induced lesions of the corticopetal cholinergic neurons of the basal forebrain on divided attention were assessed in rats. Divided attention was measured using an operant version of the crossmodal divided attention paradigm. This task utilized the propositional response rules of visual and auditory conditional discriminations. Presentation of only visual or only auditory stimuli constituted performance under the condition of modality certainty. Conversely, the presentation of a randomized sequence of all possible stimuli represented the condition of modality uncertainty and was hypothesized to tax processing capacity. A single session was composed of two unimodal blocks of trials (20 trials each), followed by a bimodal block of 60 trials. Animals were extensively trained in this task and baseline performance was characterized by high response accuracy (> 80%) in both conditions. Compared to unimodal trials, the response latencies in the bimodal block of trials were 160 ms longer. The lesion of the cholinergic system increased the response latencies exclusively under the condition of modality uncertainty. The extent of the lesion-induced decrease in cortical cholinergic fiber density correlated highly with the differences between uni- and bimodal response latencies. These results demonstrate a lesion-induced decrease in processing capacity and a preservation of response accuracy at the cost of response latency, i.e., a speed-accuracy tradeoff. Cortical acetylcholine is suggested to mediate the regulation and allocation of processing resources.