Negative slow potentials were recorded from the cortex of male rats in a passive food reward paradigm. These event-related slow potentials developed during the 3 s interval between a warning auditory cue (S1) and automatic food presentation (S2). The effects of the muscarinic antagonist atropine were examined on the slow potential responses. Atropine (0.1-0.4 mg/kg s.c.) produced a dose-related reduction in the magnitude of the slow potential responses when compared with pre-drug saline control values. The quaternary analogue of atropine, methylatropine (0.4 mg/kg s.c.), which does not enter the brain readily, had no effect on the slow potential responses. These results suggest that a central muscarinic-cholinergic component may be involved in the generation of the slow potential responses in the rat.