Abstract Brain cell swelling is a consequence of Scizure, ischemia or excitotoxicity. Changes in light reflectance from cortical surface are now used to monitor brain activity but these intrinsic signals are poorly understood. The objectives of this study were first, to show that changes in light transmittance were correlated with cell volume and second, to image increases in light transmittance as they related to neuronal activation. Transverse hippocampal slices from the rat were used for the study. Brief exposure (4–6 min) to hypo-osmotic artificial cerebrospinal fluid (−40 mOsm) elevated light transmittance consistently and reversibly in most regions of the slice and particularly in CA1 dendritic regions. Neither zero-Ca 2+ artificial cerebrospinal fluid nor tetrodotoxin altered the transmittance increase and its subsequent reversal, suggesting that it was dependent on osmolality but independent of synaptic transmission and neuronal firing. The amplitude of the CA1 population spike evoked from Schafler collaterals increased concomitantly with the hypo-osmotic increase in light transmittance, providing evidence that the extracellular tissue resistance increased. Hyper-osmotic artificial cerebrospinal fluid (+40 mOsm) containing impermeant mannitol consistently lowered light transmittance and the amplitude of the population spike. Glycerol (+40mOsm), which is cell permeant, did not have an affect. Taken together these observations indicate that osmotic challenge alters light transmittance by inducing changes in cell volume. Transmittance increases induced by hypo-osmotic artificial cerebrospinal fluid or 10μ M kainate were small in the CA1 cell body region compared to dendritic regions. Similarly, orthodromic stimulation of axons terminating in stratum oriens or in stratum radiatum evoked transmittance increases only in their respective postsynaptic areas. In contrast, the cell body region and its adjacent proximal-apical dendrites (both sites of action potential initiation) could display dramatic increases in light transmittance upon brief exposure to 20 mM K +. The response, which may represent neuronal damage, was blocked in tetrodotoxin. Antidromic stimulation evoked a weak response in these same proximal areas. We conclude that activity-dependent increases in light transmittance across brain slices primarily reveal glial and neuronal swelling associated with excitatory synaptic input and action potential discharge. The signal can be imaged in real time to reveal neuronal activation, not only among hippocampal areas, but among neuronal regions. Cell swelling is a known consequence of excessive neuronal discharge. Therefore, the imaging of changes in light transmittance across brain slices should prove useful in monitoring epileptiform and excitotoxic states.