Apparatus is provided that includes a parenchymal electrode, configured to be implanted in brain parenchyma of a subject identified as at risk of or suffering from Alzheimer's disease; and a ventricular electrode, configured to be implanted in a ventricular system of a brain of the subject. Control circuitry is configured to drive the parenchymal and the ventricular electrodes to clear a substance from the brain parenchyma into the ventricular system. Other embodiments are also described.