Cholinergic receptor antagonists are commonly used to model attentional and mnemonic impairments associated with neuropsychiatric disorders such as Alzheimer's disease. However, few studies have systematically assessed the effects of these drugs following manipulations that affect attention or working memory within the same task. In the present experiment, rats were trained to discriminate visual signals from "blank" trials when no signal was presented. This task was modified to include retention intervals on some trials to tax working memory. During standard task performance, rats received systemic injections of the muscarinic receptor antagonist, scopolamine, or of the nicotinic receptor antagonist, mecamylamine. A second experiment tested the effects on this task of co-administering doses of scopolamine and mecamylamine that, when administered alone, did not significantly affect task performance. Scopolamine (0.3 and 1.0 mg/kg) decreased detection of 500 ms signals but did not affect accurate identification of non-signals. Scopolamine did not differentially affect performance across the retention interval. Elevated omission rates were associated with high doses of scopolamine or mecamylamine. Combination drug treatment was associated with decreased signal detection and elevated omission rates. Collectively, the data suggest that muscarinic and nicotinic receptor antagonists do not exclusively impair working memory.