The Na(+)-regulating and vasoactive peptide hormone atrial natriuretic peptide (ANP) has been localized in cultured glial cells from neonatal rat brain but has not been reported in situ in adult brain. In the present study, immunohistochemical localization of ANP in astrocytes of adult canine brain was obtained using an antibody against rat ANF IV, a 25-amino acid fragment identical to low molecular weight brain ANP. ANP-like immunoreactivity was localized in widely dispersed astroglia throughout the canine brain, including Types I and II, in addition to some astrocytes of the glia limitans (GL).