Using NADPH-diaphorase (NADPH-d) histochemistry, the expression of nitric oxide synthase (NOS) was studied in the rat brain 1 week after kainate-induced status epilepticus. Major changes were observed in the hippocampi of epileptic animals, especially a loss of NADPH-d positive fibres in the periphery of degenerative pyramidal cells, the survival of NOS-containing interneurones in the dentate hilus, a different pattern of NADPH-d staining in lesioned areas, probably corresponding to the expression of inducible NOS by glial cells and an increased staining of the vasculature. These different sources of NO may exert different functions in the epileptic focus.