Spatial memory can be studied in the mouse by recording from individual pyramidal cells in the hippocampus. The firing pattern of these "place" cells create an internal representation of the animal’s location within its surrounding. A mouse is attached to a recording cable and placed inside a cylinder (49 cm in diameter by 34 cm high). The other end of the cable goes to a 235-channel commutator attached to a computer-based spike-discrimination system. The cable is also used to supply power to a light-emitting diode mounted on the headstage the mouse carries. The entire apparatus is viewed with an overhead TV camera whose output goes to a tracking device that detects the position of the mouse. The output of the tracker is set to the same computer used to detect spikes, so that parallel time series of positions and spikes are recorded. The occurrence of spikes as a function of position is extracted from the basic data and is used to form two-dimensional firing-rate patterns that can be numerically analyzed or visualized as color-coded firing-rate maps. Dark areas indicate regions in the circular enclosure in shich the cells fires at high rates.