NADPH-Diaphorase has recently been shown to be identical to nitric oxide synthase in brain neurones. In the intact brain, NADPH-diaphorase is only present in selected populations of neurones and is not detectable in glia. However following a lesion in the mouse retrosplenial cortex, activated astrocytes display intense NADPH-diaphorase activity throughout their cytoplasm. After a control saline injection, NADPH-diaphorase-positive activated glia are only observed in damaged tissue immediately surrounding the injection site, but when kainic acid is injected unilaterally, activated astrocytes occur in the hippocampal formation bilaterally. This indicates that astrocytes activated by intense neuronal activity, as well as by direct mechanical damage, express high levels of NADPH-diaphorase.