There is a unidirectional, ipsilateral and monosynaptic projection from the hippocampus to the prefrontal cortex. The cognitive function of hippocampal-prefrontal cortical circuit is not well established. In this paper, we use muscimol treated rats to investigate the roles of the hippocampal-prefrontal cortical circuits in spatial working memory, as assessed with a delayed spatial alternation task. First of all, the effect of muscimol on EEG power of infusion area was observed for confirmation of the dosage of muscimol to inhibit the function of infusion area. The results show that the EEG power of the ventral hippocampus and the prelimbic area of the prefrontal cortex were inhibited by local infusion of muscimol (0.5 microg in 0.25 microl PBS) into the above areas, respectively. Delayed alternation performance was significantly impaired when muscimol at this dosage was infused (1) bilaterally into the ventral hippocampus, (2) bilaterally into the prelimbic area, (3) unilaterally into the ventral hippocampus and simultaneously contralaterally into the prelimbic area. Infusion of muscimol either unilaterally into the ventral hippocampus or unilaterally into the prelimbic area did not impair delayed alternation performance. The present results suggest that any structures in this circuit is damaged or inhibited bilaterally, the spatial working memory will be disrupted. It means the hippocampal-prefrontal cortical circuit plays an important role in spatial working memory.