In the trunk of higher vertebrates, the neural crest (NC) cells remain temporarily within the dorsal portion of the neural tube after fusion of the neural folds; shortly thereafter they emigrate, invading surrounding spaces and tissues. One of the factors postulated to be important in the initiation of migration of NC cells is the disruption of the basal lamina (BL) over the dorsal portion of the neural tube. It has been assumed by many that the BL must be discontinuous in order that the NC cells can leave the neural tube; and indeed, experiments performed in our laboratory, and by others, have shown that NC cells cannot penetrate an intact BL. Therefore, we have undertaken a systematic ultrastructural study to evaluate the condition of the BL during neural fold elevation and NC cell emigration. Our results show that: (i) BL surrounding the neural epithelium (NE) becomes progressively more extensive from neural fold to migratory stages. It first forms on the lateral portion of the neuroepithelium of the neural folds and then extends ventrally into the region adjacent to the notochord; (ii) BL becomes continuous beneath the epidermal ectoderm (EE) that overlies the NC cell region only during the terminal stages of NC cell emigration; (iii) BL does not form over the dorsal portion of the neural tube until NC emigration is terminated; and (iv) the morphology of the BL changes as development proceeds. We conclude that absence of a BL over the premigratory NC cell population in the trunk of mouse embryos is a necessary but not a sufficient condition for emigration to take place.