AbstractStimulation of guanylate cyclase in vitro by atrial natriuretic factor (ANF) or sodium nitroprusside was studied in rat brain tissue slices biochemically as well as by means of cyclic guanosine monophosphate (cGMP) immunocytochemistry. The ANF‐responsive, cGMP‐producing cells were studied in the olfactory bulb, the septal area, the hippocampus, the medial amygdala, and the medial preoptic area. These cells, having the ANF‐stimulated particulate guanylate cyclase, were characterized as astroglial cells on the basis of their glial fibrillary acidic protein (GFAP) immunostaining, although not all astroglial cells in these areas could be identified as cGMP‐immunoreactive cells. Sodium nitroprusside‐stimulated soluble guanylate cyclase activity was demonstrated in neuronal cell bodies and varicose fibers and was associated with blood vessel walls.Upon maturation, a significant decrease in cGMP production was found after stimulation by 100 nM ANF‐(103–126) in the olfactory bulb, the medial amygdala, and the hippocampus, but not in the septal area; no change was found in these areas in cGMP content after stimulation of cGMP production by 10 μM sodium nitroprusside. Via cGMP immunocytochemistry, no qualitative differences were seen in the ANF‐responsive, cGMP‐producing cel