Coordinated activity of spatially distributed neuronal networks is thought to play important role in brain functions. We used CP55940, an agonist of cannabinoid receptor, to investigate the effect of the drug on hippocampal and prefrontal network oscillations and interactions in rats at rest and during spatial working memory performance. Simultaneous tetrode recordings revealed significant dose-dependent reduction in 0.1-30 Hz low frequency local field potential power in the hippocampus contrasted by decreases in 30-100 Hz power in the prefrontal cortex. During decision-making phases of the spatial working memory task, CP55940 disrupted theta-frequency coherence of hippocampal-prefrontal network activity which correlated with reduced accuracy of task performance and was associated with impaired phase-locking of prefrontal single unit firing to the hippocampal theta rhythm. These results suggest that cognitive deficits caused by exogenous activation of brain cannabinoid receptors may be attributed to disrupted theta-frequency coordination of activity between the hippocampus and the prefrontal cortex.