THE biological functions of nitric oxide in the neuronal system remain controversial. Using a novel fluorescence indicator, DAF-2 DA, for direct detection of NO, we examined both acute rat brain slices and organotypic culture of brain slices to ascertain NO production sites. The fluorescence intensity in the CA1 region of the hippocampus was augmented, especially after stimulation with NMDA, in acute brain slices. This NO production in the CA1 region was also confirmed in cultured hippocampus. This is the first direct evidence of NO production in the CA1 region. There were also fluorescent cells in the cerebral cortex after stimulation with NMDA. Imaging techniques using DAF-2 DA should be very useful for the clarification of neuronal NO functions.