Embryonic septal tissues grafted alongside the hippocampal formation in neonatal rat hosts were used to test for the presence of competitive interactions between developing native axons and those from the septal grafts. When native septal afferents to the target hippocampal formation are removed at the time of implantation, axons from septal grafts visualized with acetylcholinesterase (AChE) histochemistry enter the appropriate laminar zones in the host hippocampus. The pattern of septal implant growth differs markedly when native septohippocampal fibers are left intact. Under these conditions, implant-associated AChE staining is intense in the cortical area surrounding the graft but little or no reaction product is present in the host dentate gyrus and hippocampus proper. A similar result is obtained when implant viability is enhanced by introducing it 3 days after the cavity is made. These data illustrate an apparent hierarchy in competitive interactions for developing septal fibers. Septal afferents can inhibit the growth of other septal fibers and commissural/associational (c/a) axons can exclude septal fibers from their terminal field. Septal fibers and entorhinal afferents will develop concurrently.