Mouse embryos from embryonic days 8.5-10.5 (E8.5-E10.5) were fixed and labeled with an antibody to neuron-specific class III beta-tubulin (Moody et al., 1987; Lee et al., 1990a,b) to reveal the first neurons, axons, and tracts in the brain. They were studied in whole-mounts and in light microscopic sections. Some conclusions were checked by labeling tracts in older embryos (E11.5 and E12.5) with the lipophilic dye 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine. The first immunoreactive cells appeared at E8.5, prior to neural tube closure, in the neural plate immediately caudal to the optic vesicle. Cells along the dorsal midline of the mesencephalon issued the first axons, on E9.0; the cells were the mesencephalic nucleus of the trigeminal nerve, and the axons formed its descending tract. The tract reached the level of the trigeminal ganglion by E10.0 but did not enter the ganglion until after E12.5. On E9.5, the number of labeled cells and axons in the alar plate of the presumptive diencephalon and mesencephalon had increased substantially, and many of the rostral ones coursed into the basal plate to enter longitudinal tracts there. Two tracts originated from cells in the basal plate: the tract of the postoptic commissure (from the base of the optic stalk to the level of the cephalic flexure) and the medial longitudinal fasciculus (from the level of the cephalic flexure caudally through the mid and hind-brains). By E10.0, a small mammillotegmental tract paralleled the tract of the postoptic commissure, but immunolabeling was so widespread that discrete tracts were impossible to discern in the presumptive diencephalon and mesencephalon. The more rostral regions remained lightly labeled. In the cerebral vesicle, the presumptive cerebral cortex, the first immunoreactive cells appeared at E10.0; they had multiple processes oriented parallel to the pia, and were identified as the Cajal-Retzius cells. By E10.5, no tracts had formed in the cerebral vesicle. All the tracts formed by E10.0 were superficial, in the subpial lamina. Those that can be identified in the adult brain are very deep structures. These results are compared with previous descriptions of the embryonic brains of amphibians, fish, birds, and other mammals, including humans.