Nitric oxide (NO), a free radical gas, has recently been recognized as an important messenger molecule having a neurotransmitter-like function. Studies on the localization of the enzyme synthesizing NO (NO synthase-NOS) have indicated its presence in almost all parts of the brain with a prevalence in the cerebellum. From recent experimental investigations it is apparent that NO might meet the essential criteria to function as a retrograde messenger for Long-term potentiation in hippocampal cells, a process known to be involved in mammalian learning and memory. However, within the hippocampus NO is present in a few cell types which probably form the local neuronal circuit. Thus NO might function as a paracrine factor rather than a retrograde messenger in the hippocampal neurons.