Fetal mouse raphe and hippocampal tissue (from embryos with crown-rump length (CRL) 13-16 mm) was transplanted into adult and aged isogenetic mice to study the growth of serotonergic fibers between host and donor tissue. A specific antibody against serotonin (5-HT) was used to immunocytochemically visualize 5-HT containing cell bodies and fibers. Unilateral fetal transplants into the hippocampi of adult (4-6 mo.) or aged (24 mo.) mice matured and sent out processes which very densely innervated the transplant tissue itself and extended into the host hippocampus. The termination of these fibers was consistent with the known 5-HT-hippocampal lamination pattern in normal animals. Qualitative comparisons suggested that the density of outgrowth into adult hippocampus was greater than into aged hippocampus. Conversely, adult 5-HT neurons send sprouts into fetal hippocampal tissue transplanted into the lateral ventricle. Therefore, immunocytochemical procedures can be used to monitor outgrowth from the fetal tissue to the host and ingrowth from the adult host to the fetal tissue. Furthermore, the apparent normal 5-HT lamination pattern produced by fetal raphe axons in adult hippocampus is consistent with reports that neuronal transplantation is effective in reversing the anatomical and behavioral deficits produced by homotypic denervation of a terminal field.