This protocol describes a one-photon method to detect the time course and the spatial distribution of intracellular sodium ion concentrations [Na(+)](i), which result from action potentials and synaptic activity in different regions of neurons in brain slices. The one-photon technique has been applied to several different cell types and can likely be applied to other neurons in brain slices, particularly those that are relatively flat. Imaging of [Na(+)](i) changes is much less common than imaging of [Ca(2+)](i) changes, in part because typical signals are much smaller and more difficult to detect. However, with careful experiments, accurate and interesting results can be obtained. In this protocol, a cell is loaded with an indicator that responds to Na(+), the cell is stimulated, and the spatial and temporal characteristics of the fluorescence changes are recorded with an appropriate detector or camera.