Different responses evoked by stimulation of contralateral hippocampus at various levels are examined in new-born rabbits up to 1 month of age. Commissural fibres coming from the CA3-CA4 reach the contralateral regio superior (CA1) essentially at the st. oriens levels. CA1 responses elicited by stimulation of the contralateral CA1 in the st. oriens (commissural CA3 fibres) or in the st. radiatum (Schaffer's collaterals) are present at any age. For the regio inferior (CA3-CA4), the distribution of commissural fibres is more diffuse. Before 5--6 days, CA3-CA4 fields do not respond to a contralateral stimulation and do not give contralateral responses, showing that fibres and cells are not yet organized.