The putative involvement of the innervation in determining the sites of dental development was investigated by intra-ocular homografting and organ culture methods. Mandibular arches were dissected from 9 and 10-day-old (E9-E10) mouse embryos and grafted to the anterior eye chambers of homologous adult mice for 12-14 days. There was no significant difference in the incidence of tooth formation between grafts in which the trigeminal ganglion was included (n = 82, 68 per cent) or excluded (n = 72, 65 per cent). A parallel in vitro study in which E9 and E10 mouse embryo mandibular arches were cultured in the absence of trigeminal innervation showed that definitive tooth germs were formed during the 7-9 day culture period. It is concluded that innervation plays no part in determination of tooth development. In further experiments with E9 and E10 material, the complete mandibular arch, the hemimandibular arch and the ventral midline region of the mandibular arch (including the median epithelial isthmus) were each grafted for a period of 21 days. Alizarin-red whole-mount staining of recovered grafts revealed that bone had been deposited and mineralized in the majority of grafts of all types. Incisor and molar teeth with near normal crown shapes developed in grafts of complete mandibular arches. Hemimandibular arches gave rise almost exclusively to molars. Grafts of the E10 ventral midline region gave rise exclusively to incisors, but the same graft performed at E9 did not produce teeth. It is concluded that incisor primordia are localized in or near the median epithelial isthmus (and are thus destroyed or damaged when the mandibular arch is hemisected) and that incisors are not determined until E10. At E9 the odontogenic neural crest has yet to complete its ventrad migration into proximity with presumptive incisor epithelium. By this time, however, odontogenic crest and presumptive molar epithelium have already reached juxtaposition and molar primordia are fully competent. In grafts of the frontonasal and maxillary processes made at E10, prior to merging of their respective mesenchymes, frontonasal processes gave rise exclusively to incisors, whereas maxillary processes gave rise exclusively to molars.