Atrial natriuretic factor (ANF)-responding, cyclic guanosine monophosphate (cGMP)-producing cells were visualized in the hippocampus of the rat applying cGMP immunocytochemistry to hippocampal tissue slices incubated in vitro. These cells were found scattered throughout the hippocampus with their highest density in the hilus fascia dentata. Staining with an antibody against glial fibrillary acidic protein showed a completely different pattern, suggesting a non-glial nature of the ANF-responding cells. cGMP accumulation assayed biochemically was observed already at 10 microM ANF and was sensitive to inhibition by Methylene blue. The immunocytochemical data were fully supported by the biochemical measurements of cGMP under these conditions. Our results show for the first time a response to ANF of individual, cGMP-producing cells in the CNS of the rat.