Nitric oxide (NO) mediates cell-cell signalling in the brain and stimulates cyclic GMP (cGMP) production in target cells. We have used NADPH-diaphorase (reduced nicotinamide adenine dinucleotide phosphate-diaphorase) histochemistry to identify NO-producing neurones and cGMP immunohistochemistry to locate the targets of NO in rat cerebellum. NADPH-diaphorase staining was prominent in granule cells and in the molecular layer. cGMP immunostaining in cerebellar slices stimulated with the NO donors, nitroprusside and SIN-1, was found in granule cells, glomeruli, fibres, Bergmann glia and in other astrocytes. The results provide visible evidence that NO mediates neuron-neuron and neuron-glia communication.