CD38 is a transmembrane type II glycoprotein expressed in the brain (1). CD38 is an important source of cyclic ADP-ribose (1,2), which can act as a second messenger in the downstream calcium signalling pathways mediated by neurotransmitter receptors (1,3). The marked differences in regional distribution of CD38 had not been determined until our recent study (4). In the mouse brain, CD38 mRNA level and enzymatic activity were greatest in the hypothalamus, followed by the posterior pituitary (4). We also proposed that CD38 plays an essential role in oxytocin secretion from somatodendrites and neurohypophysial terminals of hypothalamic neurons (4,5), with profound consequences in social and nurturing behaviours (4,6). Immunohistochemical visualisation of CD38 is a powerful technique to confirm the brain regional localisation and cell type specificity of CD38 at the protein level. However, there have been only a few reports on this issue, and those published to date had very low resolution (7,8), except for recent studies using double-labelling of CD38 and glial fibrillar acidic protein, synaptotagmin or synaptophysin (9-11). Here, we report a detailed procedure to observe the localisation of CD38 and oxytocin in the mouse hypothalamus using a double-labelling immunofluorescent staining method by confocal microscopy.