The activity of single fibers from the saccular nerve has been recorded in isolated head preparations in the frog (Rana esculenta). Nearly all the fibers present an activation or a depression of their activity during the tilting movement in a given direction. This is considered to correspond to the existence of two anatomical populations of haircells in the saccular macula which differ in their functional polarization. The activity of only some units (about 40%) remained increased or decreased when the head was in a tilted position for 4–5 min. This shows that the sacculus is sensitive to the direction of gravity.