Fifteen seconds after intracarotid injection of [125I]lysine vasopressin, [3H]oxytocin, [3H]H2O or [3H]inulin, the distribution of radioactivity in 18 regions of the rat brain and in the anterior pituitary was determined. Comparing the concentration of the different tracers (in percent of injected radioactivity per g brain tissue), it is assumed that a small amount of the labeled neurohormones is taken up by the brain, indicating a penetration of the blood-brain barrier and/or an accumulation within the structures of the barrier system.