The commissural fibres of hippocampus and dentate gyrus were studied by the method of horseradish peroxidase (HRP) in the guinea pig. The results are as follows:(1) The superior region (CA_1)of hippocampus sends commissural fibres to the same field of the contralateral side.(2) The inferior region (CA_3) of hippocampus sends commissural fibres to all fields of hippocampus and dentate gyrus of the contralateral side.(3) The cells of polymorphic layer of the dentate gyrus send commissural fibres to the dentate gyrus or the inferior region (CA_3) nearby the dentate gyrus of the contralateral side, or perhaps to both of them.(4) The superior region of hippocampus receives commissural fibres which have remarkable degree of symmetry, and this is probably also true of the dentate gyrus, but the inferior region of hippocampus is not likely so.