The cerebral cortex, cerebellar cortex and hippocampus of adult mice were studied by light and electron microscopy of tissue sections. Dark neurons were well stained with nuclear fast red, and clearly distinguishable from light neurons. The dark neurons were classified into active and resting types. Active types were characterized by their well developed Golgi's complexes. Resting types contained poorly developed Golgi's complexes. These findings indicate that active dark neurons are converted into the light neurons via resting dark neurons.