Cognitive impairments in Alzheimer's disease (AD) have been associated with alterations in neuronal oscillatory activity, of which hippocampal theta and gamma oscillations are essential for the coordination of neuronal networks during cognitive functions. Cognitive deterioration in AD is delayed by symptomatic treatment with donepezil and other acetylcholinesterase inhibitors (AChEIs). However, the efficacy of symptomatic monotherapy is insufficient. Combining 5-HT receptor antagonists with AChEIs represents a promising new approach for symptomatic treatment of AD. The selective 5-HT₃ receptor antagonist ondansetron decreases the activity of interneurons with a concomitant increase in the activity of pyramidal neurons in the hippocampus of freely moving rats. Additionally, 5-HT₃ receptor antagonism modulates acetylcholine release in rat cortex and hippocampus. We investigated the effects of ondansetron alone and in combination with donepezil on hippocampal oscillations using in vivo electrophysiology. Neuronal network oscillations were recorded in the dorsal hippocampus during electrical stimulation of the brainstem pedunculopontine tegmental nucleus in urethane-anaesthetised rats. In addition, potential pharmacokinetic interactions between donepezil and ondansetron were assessed. Ondansetron alone did not affect hippocampal network oscillations. Donepezil dose-dependently increased hippocampal theta and gamma power during PPT stimulation. Ondansetron (0.3 mg/kg, i.v.) potentiated theta and gamma responses to 0.2 mg/kg donepezil and prolonged theta and gamma responses to 0.3 mg/kg donepezil. These effects could not be attributed to pharmacokinetic interactions between the compounds. This study demonstrates that ondansetron potentiates the effects of donepezil on elicited neuronal oscillations and suggests that 5-HT₃ receptor antagonists may be beneficial as adjunctive therapy to AChEIs for the symptomatic treatment of cognitive deficits in AD.