Imaging analysis techniques were used to examine changes in the intrinsic optical properties in hippocampal brain slices that occurred during synaptic activity evoked by Schaffer collateral stimulation in CA1. Repetitive synaptic activity was associated with an increase in light transmission in the synaptic region in stratum radiatum. The effect was seen at wavelengths of light between 450 and 800 nm but was of greater amplitude at longer wavelengths. Blocking synaptic transmission with either Ca(2+)-free EGTA perfusate or kynurenic acid (an excitatory amino acid antagonist) blocked the optical signal, indicating that it resulted from postsynaptic activation of the cells and was not due to presynaptic fiber volleys or transmitter release alone. Because the optical changes were blocked by reducing extracellular Cl- (by replacement with gluconate) or by furosemide (an anion transport inhibitor), increased Cl- transport (conceivably Na-K- 2Cl cotransport) may generate these signals possibly by causing cellular swelling and thereby less light scattering. These optical changes were not blocked, however, by bicarbonate-free solution, indicating that bicarbonate transport may not be involved. Changes in the intrinsic optical signal could be related to glial swelling due to K+ released during neuronal activity because high-K(+)-induced swelling of cultured astrocytes is blocked by furosemide and low-Cl- solution. Intrinsic optical signals of neuronal tissue should be considered when voltage- or ion-sensitive dyes are used.