Neurovascular coupling is the process by which active areas of the brain signal to blood vessels to increase local perfusion and oxygen delivery.  A variety of signaling molecules and mechanisms have been proposed and tested experimentally.  Potassium is one potential signal: active neurons release potassium into the extracellular space, potassium readily diffuses, and potassium can cause changes in blood vessel diameter.  I will present theoretical models of potassium diffusion in the brain.