In the present study, the ability of 5-hydroxytryptamine-4 (5-HT 4 ) receptors in the hippocampus to enhance locomotor activity in rats was investigated by local infusion via microdialysis probes. The local infusion of 5-HT bilaterally into the striatum did not alter rat motor activity. The local infusion of 1.0 mM 5-HT into the bilateral hippocampus, but not lower doses, significantly increased motor activity as compared with the baseline values or the control rats. During the day hours (0700 -1900, light on), the local infusion of either 5-HT 4 agonist, 5-MeOT (100 M) or mosapride (10 M), but not in their lower concentrations, into the bilateral hippocampus significantly increased motor activity as compared with the baseline values or the control rats. Almost all increased motor activity was normal forward locomotion. This 5-MeOTinduced hyperlocomotion was completely reversed by the combined infusion of a 5-HT 4 antagonist, either GR125487D (100 M), SB204070 (100 M) or RS23597-190 (100 M). During the night hours (1900 -0700, light off), the local infusion of either SB204070 (100 M) or RS23597-190 (100 M), but not in their lower concentrations, into the bilateral hippocampus significantly decreased rat motor activity and inhibited rat nocturnal hyperactivity. These hypoactivities during the night hours induced by 5-HT 4 antagonist were reversed by the combined infusion of a 5-HT 4 agonist, 5-MeOT (100 M). The present study demonstrates that the serotonergic neurons projecting to the hippocampus, but not to the striatum, modulate rat locomotor activity by stimulating 5-HT 4 receptors in the hippocampus. Hippocampus 2002;12:304 -310.