Polybrominated diphenylethers (PBDEs) are widely used as flame retardants. Significant amounts of PBDEs are present in the milk of lactating women. The possible neurotoxic effects of PBDEs are not well known. Perinatal exposure to PBDEs affects both motor and cognitive functions by mechanisms that remain unclear. Some types of learning depend on N-methyl-d-aspartate receptor activation, which increases intracellular calcium that binds to calmodulin and activates nitric oxide synthase, increasing nitric oxide formation that activates guanylate cyclase, increasing cGMP formation. Part of this cGMP is released to the extracellular fluid. We studied whether prenatal exposure of rats to PBDE99 alters the function of this glutamate–nitric oxide–cGMP pathway in rat brain in vivo. At 10 weeks of age, rats treated with PBDE99 showed increased function of the glutamate–nitric oxide–cGMP pathway in brain in vivo, as assessed by microdialysis in freely moving rats. The increased function of the pathway was reproduced in primary cultures of cerebellar neurons prepared from rats prenatally exposed to PBDE99 as well as in neurons cultured from normal rats and treated in vitro with PBDE99. Increased calmodulin content and activation of soluble guanylate cyclase by nitric oxide contributed to the increased function of the pathway.