The study investigates the role of the hippocampus in spatial memory and object recognition memory in rats. It shows that the hippocampus is crucial for both types of memory, but spatial memory requires more hippocampal tissue than recognition memory. Spatial memory was impaired after bilateral dorsal hippocampal lesions that damaged 30–50% of the total hippocampal volume, and performance worsened as lesion size increased. In contrast, object recognition memory was intact after dorsal hippocampal lesions that damaged 50–75% of the total hippocampal volume, but was impaired only after larger lesions that damaged 75–100% of the hippocampus. Ventral hippocampal lesions that damaged approximately 50% of the total hippocampal volume impaired spatial memory but did not affect object recognition memory. These findings suggest that the hippocampus is important for both spatial and recognition memory, but spatial memory requires more hippocampal tissue than recognition memory. The hippocampus is part of a system of anatomically related structures in the medial temporal lobe important for mammalian memory. Damage to this region impairs performance on various learning and memory tasks. The hippocampus is the final stage of convergence in the medial temporal lobe, receiving projections from the perirhinal and parahippocampal/postrhinal cortices, as well as the entorhinal cortex. It is thought to be especially important for tasks that require relating or combining information from multiple sources, such as spatial memory tasks. In contrast, tasks that do not have such requirements, such as recognition memory for single items, may be supported by cortex adjacent to the hippocampus. Efforts to test these ideas have led to mixed results. Some studies report that hippocampal lesions impair recognition memory, while others show that recognition memory is largely spared. However, it is unequivocal that hippocampal damage severely impairs spatial memory. The study tested this prediction by examining the effects of hippocampal lesions of varying sizes on spatial memory and object recognition memory in rats. The results showed that spatial memory performance requires more hippocampal tissue than recognition memory. The findings support the conclusion that the hippocampus is important for both spatial memory and recognition memory. These results suggest that some earlier reports have not found recognition memory to be impaired after hippocampal lesions because the damage may not have been sufficiently complete to reveal a deficit.The study investigates the role of the hippocampus in spatial memory and object recognition memory in rats. It shows that the hippocampus is crucial for both types of memory, but spatial memory requires more hippocampal tissue than recognition memory. Spatial memory was impaired after bilateral dorsal hippocampal lesions that damaged 30–50% of the total hippocampal volume, and performance worsened as lesion size increased. In contrast, object recognition memory was intact after dorsal hippocampal lesions that damaged 50–75% of the total hippocampal volume, but was impaired only after larger lesions that damaged 75–100% of the hippocampus. Ventral hippocampal lesions that damaged approximately 50% of the total hippocampal volume impaired spatial memory but did not affect object recognition memory. These findings suggest that the hippocampus is important for both spatial and recognition memory, but spatial memory requires more hippocampal tissue than recognition memory. The hippocampus is part of a system of anatomically related structures in the medial temporal lobe important for mammalian memory. Damage to this region impairs performance on various learning and memory tasks. The hippocampus is the final stage of convergence in the medial temporal lobe, receiving projections from the perirhinal and parahippocampal/postrhinal cortices, as well as the entorhinal cortex. It is thought to be especially important for tasks that require relating or combining information from multiple sources, such as spatial memory tasks. In contrast, tasks that do not have such requirements, such as recognition memory for single items, may be supported by cortex adjacent to the hippocampus. Efforts to test these ideas have led to mixed results. Some studies report that hippocampal lesions impair recognition memory, while others show that recognition memory is largely spared. However, it is unequivocal that hippocampal damage severely impairs spatial memory. The study tested this prediction by examining the effects of hippocampal lesions of varying sizes on spatial memory and object recognition memory in rats. The results showed that spatial memory performance requires more hippocampal tissue than recognition memory. The findings support the conclusion that the hippocampus is important for both spatial memory and recognition memory. These results suggest that some earlier reports have not found recognition memory to be impaired after hippocampal lesions because the damage may not have been sufficiently complete to reveal a deficit.