Palatal rugae began to develop in the mouse, before the elevation of the palatal shelves, as localized regions of epithelial proliferation and thickening. Subsequently, fibroblasts and collagen fibres accumulated in the connective tissue subjacent to the thickened epithelium and later assumed a distinctive orientation, the fibres running anteroposteriorly within the core and in concentric curves across the base of each ruga. The role of collagen in rugal morphogenesis was examined after inhibiting its formation by feeding the lathyritic agent beta-aminopropionitrile to pregnant females. This substance markedly affected the eventual height of the rugae at birth, confirming the importance of collagen in rugal development.