The NMDA receptor, which has been implicated in memory formation, is noncompetitively blocked by ketamine. The present study examines the effect of ketamine (0, 3, 6, 12, and 25 mg/kg body wt; ip) on tunnel maze and water maze performance in Wistar rats. In the hexagonal tunnel maze (HTM) high doses of ketamine (12 and 25 mg/kg) decreased locomotor activity. Moreover, ketamine induced perimeter walking (6, 12, and 25 mg/kg) and attenuated exploratory efficiency (25 mg/kg). When the HTM was converted into a modified six-arm radial maze, ketamine impaired short-term but not long-term memory. In the Morris water maze, rats injected with ketamine (12 and 25 mg/kg) acquired a spatial navigation task more slowly than controls. When the escape platform was removed, the drug-treated rats did not preferentially search for it in the area where the platform had been during the acquisition phase. However, when the escape platform was visible, no differences in the performance of ketamine-treated and control rats could be found. In summary, ketamine seems to attenuate some but not all forms of learning in the tunnel maze and it impairs the acquisition of a spatial navigation task.