We investigated the nature of cGMP-synthesizing cells in the developing rat forebrain using cGMP-immunocytochemistry in combination with in vitro incubation of brain slices. When brain slices of immature rats, aged between 1 and 4 weeks, were incubated with sodium nitroprusside (SNP), a nitric oxide (NO) donor compound, in the presence of the phosphodiesterase inhibitor isobutylmethylxanthine (IBMX), small round cells with a few processes in and around the corpus callosum were visualized with the cGMP-antibody. The morphology and the distribution of the cGMP-positive cells were consistent with the criteria for oligodendrocytes. Furthermore, the cGMP-positive cells expressed 2'3'-cyclic nucleotide 3'-phosphodiesterase (CNPase) and gelsolin, which are marker proteins for oligodendrocytes. Therefore, we concluded that the cGMP-positive cells were oligodendrocytes. A subpopulation of the oligodendrocyte was found to be cGMP-immunoreactive also when slices were incubated in the absence of SNP. Furthermore, incubation of the slice in the presence of L-NAME, an inhibitor of NO synthase, but in the absence of SNP abolished cGMP immunostaining. In addition, some populations of neurons and astrocytes in restricted brain areas produced cGMP in response to the incubation with SNP as previously reported, whereas both ameboid and ramified microglial cells did not respond to the treatment. Atrial natriuretic peptide, a stimulator of particulate guanylyl cyclase, enhanced cGMP synthesis in astrocytes in some brain regions but not in oligodendrocytes. These findings indicate that oligodendrocytes in the immature rat brain express soluble guanylyl cyclase. No cGMP-positive oligodendrocytes were found in the mature rat brain, suggesting that cGMP may mediate signals related to myelinogenesis in the rat brain.