1. By measurement of the rate of disappearance of injected tracer thiamine from the bloodstream, a programme for the continuous injection of thiamine at a variable rate has been devized by which a steady raised level can be achieved rapidly and maintained in the circulation. By this means the flux of radioactive thiamine across the blood-brain barrier has been measured. 2. In separate experiments progressively higher levels of thiamine were maintained in the bloodstream. Evidence was obtained that the transport of thiamine across the blood-brain barrier is a carrier-mediated process which can be saturated by raised levels of thiamine. 3. The saturation of the transport process was incomplete: kinetic analysis showed that there was a non-saturable component of the transport which was probably due to passive diffusion. 4. The contribution of the non-saturable component was normally small and is probably insufficient to meet the needs of the brain for the vitamin unless the concentration of the vitamin in the blood is raised considerably above normal. 5. This two-component transport process had substantially similar kinetic parameters in different regions of the brain.