Maxillary and mandibular incisors of mice aged 5 wk were sectioned and ground along various planes, acidetched and observed by scanning electron microscopy (SEM). The general design of the enamel structure resembled rat incisor enamel with an uniserial lamellar pattern of prisms in the inner enamel and incisally directed parallel prisms in the outer enamel. The centrolabial thickness of the enamel was about 60 microns in the maxillary and about 95 microns in the mandibular incisor. The angle between prism rows and enamel-dentine junction was about 70 degrees in the maxillary and about 45 degrees in the mandibular incisor, while the angle of decussation, which increased from the enamel-dentine junction towards the outer enamel, was 50-95 degrees and 30-80 degrees respectively. The angle between outer enamel prisms and enamel surface was about 12 degrees in the maxillary and 5-15 degrees in the mandibular incisor. The outer 1/2-1/3 of the outer enamel contained iron and was more acid-resistant than the rest of the enamel. The superficial 3-5 microns was prismless with a Fe/Ca ratio of about 25/75 in the maxillary and about 10/90 in the mandibular incisor. The latter concentration of iron was insufficient to give visible pigmentation to the enamel. The extreme mesial and lateral enamel was neither typical of inner nor of outer enamel. Assuming that the length of the zone of enamel secretion is half the corresponding length in the rat, it could be calculated that ameloblasts in mouse mandibular incisors produce enamel at a rate of about 6 microns per day, about half the corresponding rate in the rat. In spite of this, the mouse mandibular incisor has a relatively thick layer of enamel, since the ameloblasts spend a relatively long time in the zone of enamel secretion due to a fairly slow eruption rate.