Auditory evoked potentials (AEPs) were recorded epidurally at vertex in the freely moving rat, yielding the prominent components P18 and N40 from the average waveform. Both components were severely attenuated at tone presentation rates faster than 1 Hz. Dependence of the vertex AEP on central cholinergic activation was assessed by comparing the effects of subcutaneous injections of the muscarinic antagonist scopolamine (0.1-1.0 mg/kg) to methyl-scopolamine (0.5, 1.0 mg/kg) to test for peripheral effects. Scopolamine produced a significant decrease in amplitude for both the P18 and N40 components in overall tests. N40 amplitude also showed a significant decrease at each dose of scopolamine, and the magnitude of the amplitude decrement was dose dependent. At 23 h after scopolamine, only N40 amplitude remained significantly depressed. There were no effects of injection of saline, or of methyl-scopolamine. These results confirm that middle-latency vertex AEPs in the rat are attenuated by central muscarinic receptor blockade, consistent with previous findings in cats.