We attempted to clarify how age-related changes in nitric oxide (NO) in the rat supraoptic nucleus (SON) influenced salivary secretion. Although the number of the Nissl-stained neurons did not change with age, age-related significant differences were detected in the cell number, cell size and reactive density of the NADPH-d-positive neurons, which is a marker for nitric oxide synthase. We concluded that the age-related increased production in NO might be a factor that contributes to age-related inhibition of salivary secretion.