Abstract A four-electrode method of chronically measuring impedance of less than 1 mm 3 parts of brain in unrestrained rats is described. The method is usually good from about 1.5 to 3,000 cycle/sec for the absolute value of phase angle, and relative changes in magnitude of impedance can be measured from about 1.5 to 10,000 cycle/sec. Testing current of 2 × 10 −8 amp is used, which produces a signal of about 20 μv. Stability for hours and days is sometimes, but not always, achieved. In the subicular area, there is invariably an increase in magnitude of impedance of up to 25% during paradoxical sleep. In the same animal, this increase is the same during all episodes of paradoxical sleep lasting longer than 1 min, and is the same at 32 to 10,000 cycle/sec. The phase angle at 100 to 1,000 cycle/sec becomes 1 or 2 deg more negative during paradoxical sleep. There are no changes or very small changes in Ammon's horn and the fascia dentata. Because these data are fairly complete, the mechanisms which might possibly be responsible for these impedance changes can be stated specifically in semiquantitative form. Among several possibilities, a single one cannot be chosen on the basis of these impedance data alone, but from other considerations, a decreased size of interstitial space seems by far the most likely.