Experimental evidence demonstrates that cerebellar networks are involved in spatial learning, controlling the acquisition of exploration strategies without blocking motor execution of the task. Action learning by observation has been considered somehow related to motor physiology, because it provides a way of learning performances that is almost as effective as the actual execution of actions. Neuroimaging studies demonstrate that observation of movements performed by others, imagination of actions, and actual execution of motor performances share common neural substrates and that the cerebellum is among these shared areas. The present paper analyzes the effects of observation in learning a spatial task, focusing on the cerebellar role in learning a spatial ability through observation. We allowed normal rats to observe 200 Morris water maze trials performed by companion rats. After this observation training, ''observer'' rats underwent a hemicerebellectomy and then were tested in the Morris water maze. In spite of the cerebellar lesion, they displayed no spatial defects, exhibiting exploration abilities comparable to controls. When the cerebellar lesion preceded observation training, a complete lack of spatial observational learning was observed. Thus, as demonstrated already for the acquisition of spatial procedures through actual execution, cerebellar circuits appear to play a key role in the acquisition of spatial procedures also through observation. In conclusion, the present results provide strong support for a common neural basis in the observation of actions that are to be reproduced as well as in the actual production of the same actions. R ecent findings demonstrated that the cerebellum must be included in the neural substrates that process spatial information (1-4). Whereas neocortical and hippocampal regions are involved in memory for spatial and object information and in locating objects in a given environment (5-8), the cerebellum appears to be involved in the navigational system with overt procedural features that control the way to reach an object, explore a new environment, and acquire spatial knowledge in relation to personal orientation. Thus, the cerebellar function is specifically linked to ''how to find an object'' rather than ''where the object is'' (9). This evidence mainly results from experimental data reporting that in the Morris water maze (MWM), hemicerebellectomized (HCbed) rats are impaired in developing efficient exploration strategies, but not in locating platform position (3). When the task is proposed for the first time postoperatively, HCbed animals display only peripheral circling around the pool, a rather ineffective way of acquiring spatial information. Conversely, when spatial procedures are acquired by executing the MWM task preoperatively, the cerebellar lesion does not alter spatial task execution.Thus, these findings demonstrate that cerebellar circuitry is involved only (or primarily) in the acquisition of procedural components of spatial tasks and that a cerebellar le...