A method for electrolytically transecting the whole fornix and its behavioral evaluation are described. Sham controls and animals with transected fornices were run on a reversal of a position habit. Animals with the transected fornices required significantly more trials to reverse than did controls. Transecting the fornix behind the commissure is considered to be similar qualitatively to hippocampal ablation as it is commonly done in its effect on reversal. A suggestion for determining the respective involvement of the CA3–4 and CA1–2 fields of the hippocampus in various behaviors is made.