Host brain receptivity to fetal hippocampal grafts was investigated following transplantation into unilateral kainic acid (KA) lesions of adult rat hippocampus. E18-E19 hippocampal cell suspensions were labeled with rhodamine dextran amine and transplanted bilaterally into hosts at various times following the KA-lesion. After one to three and one half months survival the grafts (contained within host hippocampal slices) were analyzed using intracellular electrophysiological techniques. A nonparametric graft index was developed which assessed the overall size and distribution of the graft. Using this grading system graft development was noted to be significantly enhanced for grafts placed into hosts with KA lesions at either 2-4 days or 11-12 days following the lesion, compared to grafts placed at either 6-7 days or 27-33 days after the lesion. Also, grafts implanted at delays of either 14-16 or 28-33 days appeared to have fewer surviving cells but were more dispersed within the host brain than grafts at shorter post-lesion implant times. Synaptic responses to host stimulation were noted in most grafts. Intracellular staining of transplanted neurons showed considerable development of cell processes but atypical pyramidal cell morphology and ectopic location; numerous axons traveled into the host tissue. The time course of lesion-induced host receptivity appeared to significantly influence graft development and maturation. In this study graft survival was partially independent from cell migration. This differential effect may be due to various aspects of host brain receptivity, which in turn is influenced by the delay between the host brain lesion and grafting.