This study investigates the volume and location of hippocampal tissue required for normal spatial learning in rats. Using ibotenic acid to create bilateral symmetric lesions of varying sizes (20–100% of hippocampal volume), the researchers found that even a small transverse block (minislab) of the hippocampus, located at the septal (dorsal) pole, could support spatial learning in a water maze. Lesions of the septal pole, leaving 60% of the hippocampus intact, caused a learning deficit, despite normal electrophysiological responses, synaptic plasticity, and preserved acetylcholinesterase staining. This suggests that hippocampal-dependent spatial learning only requires a minislab of dorsal hippocampal tissue. Converging evidence indicates that functional hippocampal tissue is necessary for allocentric spatial learning and memory. Hippocampal lesions disrupt learning and retention of spatial maze tasks. Recordings from pyramidal and granule cells show firing in relation to the animal's spatial position, even when relevant cues are removed. The hippocampus may function as a distributed memory system, with adjacent cells having very different place fields and cells representing the same place found at different locations in the hippocampus. Previous studies with aspiration lesions showed that lesions exceeding 10% of hippocampal tissue affect spatial learning. However, the aspiration technique may have caused unintended fiber damage. To avoid this, fiber-sparing lesions were made with ibotenic acid. The size of the lesions was varied systematically, with spared tissue forming slabs at the septal or temporal end. Spatial learning was tested in an open-field water maze. The results showed that learning to navigate to the platform was more efficient when residual tissue was part of the dorsal hippocampus than when it was part of the ventral hippocampus. Rapid place navigation was possible with a relatively small slab of intact hippocampal tissue. There was a linear relationship between the degree of learning deficit and the amount of dorsal hippocampal tissue included in the remaining ventral tissue block. Normal learning rates were observed when dorsal tissue was included in the remaining blocks. The study also found that the necessary processing could be performed by minislabs oriented in the lamellar plane and located at either the septal pole or closer to the border between the dorsal and ventral halves of the hippocampus. This suggests that spatial acquisition may not be locked to a particular group of cells within the dorsal hippocampus. The findings indicate that spatial learning requires the dorsal but not the ventral hippocampus. The results suggest that the hippocampus may contain multiple facultative networks for spatial learning and that the hippocampal memory system has redundancy. Efficient spatial learning required only 20–40% of the total hippocampus, corresponding to about half of the portion that receives relevant sensory input. These findings support the idea that the hippocampus functions as a distributed associative memory system.This study investigates the volume and location of hippocampal tissue required for normal spatial learning in rats. Using ibotenic acid to create bilateral symmetric lesions of varying sizes (20–100% of hippocampal volume), the researchers found that even a small transverse block (minislab) of the hippocampus, located at the septal (dorsal) pole, could support spatial learning in a water maze. Lesions of the septal pole, leaving 60% of the hippocampus intact, caused a learning deficit, despite normal electrophysiological responses, synaptic plasticity, and preserved acetylcholinesterase staining. This suggests that hippocampal-dependent spatial learning only requires a minislab of dorsal hippocampal tissue. Converging evidence indicates that functional hippocampal tissue is necessary for allocentric spatial learning and memory. Hippocampal lesions disrupt learning and retention of spatial maze tasks. Recordings from pyramidal and granule cells show firing in relation to the animal's spatial position, even when relevant cues are removed. The hippocampus may function as a distributed memory system, with adjacent cells having very different place fields and cells representing the same place found at different locations in the hippocampus. Previous studies with aspiration lesions showed that lesions exceeding 10% of hippocampal tissue affect spatial learning. However, the aspiration technique may have caused unintended fiber damage. To avoid this, fiber-sparing lesions were made with ibotenic acid. The size of the lesions was varied systematically, with spared tissue forming slabs at the septal or temporal end. Spatial learning was tested in an open-field water maze. The results showed that learning to navigate to the platform was more efficient when residual tissue was part of the dorsal hippocampus than when it was part of the ventral hippocampus. Rapid place navigation was possible with a relatively small slab of intact hippocampal tissue. There was a linear relationship between the degree of learning deficit and the amount of dorsal hippocampal tissue included in the remaining ventral tissue block. Normal learning rates were observed when dorsal tissue was included in the remaining blocks. The study also found that the necessary processing could be performed by minislabs oriented in the lamellar plane and located at either the septal pole or closer to the border between the dorsal and ventral halves of the hippocampus. This suggests that spatial acquisition may not be locked to a particular group of cells within the dorsal hippocampus. The findings indicate that spatial learning requires the dorsal but not the ventral hippocampus. The results suggest that the hippocampus may contain multiple facultative networks for spatial learning and that the hippocampal memory system has redundancy. Efficient spatial learning required only 20–40% of the total hippocampus, corresponding to about half of the portion that receives relevant sensory input. These findings support the idea that the hippocampus functions as a distributed associative memory system.