The distribution of i.v. injected 109Cd within the nervous system was studied in rats 24 h and 1 week after the injection. Measurements by gamma scintillation showed a high uptake of cadmium in peripheral sensory and autonomic ganglia, whereas the uptake was low in the brain, cerebellum, and spinal cord. The accumulation of cadmium in the sciatic nerve was significantly higher than in the brain and spinal nerve roots, but lower than in ganglia. At autoradiography no labeling was seen in the major part of the brain parenchyma, but an accumulation of the metal was observed in structures outside of the blood-brain barrier (BBB), such as the hypophysis, meninges, choroid plexus and pineal gland. Within the peripheral nervous system (PNS), autoradiography showed accumulation of cadmium in the dorsal root ganglia. The results show that the distribution of 109Cd within the nervous system is correlated to regional variations in vascular permeability, blood vessels of different regions permitting penetration of different amounts of the protein-bound cadmium into the nervous tissues. The accumulation of cadmium in certain nervous structures may have relevance for some of the neurotoxicologic effects of this metal that have been demonstrated in animal experiments.