A simple and inexpensive slicer has been developed for the preparation of slices of mouse or rat brain. The instrument consists of razor blades, separated by an 0.5 mm thick polyethylene sheet (1 X 1 cm), mounted on metal screws through a hole in the center of the polyethylene sheet. Using this slicer, 6-8 uniform slices of 500-mu-m thickness were obtained from mouse or rat brain. These brain slices were incubated in a medium consisting of artificial cerebrospinal fluid for 1 h at 37-degrees-C under an oxygen atmosphere and the activities of various subcellular marker enzymes were assayed. The slice weights and the activities of the enzymes did not vary significantly in different batches of slices. Morphological evaluation of the slices revealed well-preserved neurons. Histochemical staining for mitochondrial enzymes revealed intense staining of neuronal cells and lighter staining of the white matter in all the regions examined. These slices could serve as a useful in vitro model for studying brain function and the effect of various toxicants on the brain.