These experiments have investigated selenium movement between blood and CNS in anaesthetised rats. Each animal was surgically anaesthetised and the left femoral blood vessels cannulated for blood withdrawal and solute infusion. Each rat received 75-selenium as sodium selenite infused in normal saline and experiments lasted between 5 minutes and 5 hours during which blood samples were periodically taken. At termination, the CNS was removed, regionally dissected and analysed with the plasma samples for 75-Se radioactivity by gamma-counting. Data were analysed by graphical analysis. Results showed unidirectional uptake of 75-Se into the CNS and regional differences were not found except for the hypothalamus. On average the CNS influx rate constant (Kin) was about 7 +/- 1 x 10(-5) ml/min/g. Data suggest that the 75-Se most likely entered the CNS as a free ionic form.