A new subdivision, named marginal division (MrD), consisting of spindle-shaped neurons, has been identified at the caudomedial margin of the neostriatum in the brains of the rat, cat, monkey and human. It is distinguishable from the rest of striatum by special neural connections and many intensely expressed neuropeptides and some monoamines in the fibers, terminals and neuronal somata. Three-dimensional reconstruction of the rat brain reveals that the MrD is a flat pan-shaped area between the neostriatum and globus pal-lidus. Chemical lesions of bilateral MrD in rats will result in severely impaired learning and memory functions, as was demonstrated by double blind Y-maze test. The function of MrD has been shown to be associated with learning and memory by functional magnetic resonance imaging (fMRI) technique in human brain in vivo. Functional neuronal connections are observed between the MrD and hippocampus, amygdala, as well as the basal nucleus of Meynert by chemically induced c-Fos immunohistochemi A new subdivision, named marginal division (MrD), consisting of a spindle-shaped neurons, has been identified at the caudomedial margin of the neostriatum in the brains of the rat, cat, monkey and human. It is distinguishable from the rest of striatum by special neural connections and many intensely expressed neuropeptides and some monoamines in the fibers, terminals and neuronal somata. Three-dimensional reconstruction of the rat brain reveals that the MrD is a flat pan-shaped area between the neostriatum and globus pal-lidus. Chemical lesions of bilateral MrD in rats will result in severely impaired learning and memory functions, as was demonstrated by double blind Y-maze test. The function of MrD has been shown to be associated with learning and memory by functional magnetic resonance imaging (fMRI) technique in human brain in vivo. Functional neuronal connections are observed between the MrD and hippocampus, amygdala, as well as the basal nucleus of Meynert by chemically induced c-Fo s immunohistochemi