The structure of the mesenchymal component of the palatal shelves of fetal C3H mice was studied at the time of shelf reorientation. The embryos were removed at 13.5 and 14.5 days of gestation. Specimens were either stained with PAS (Periodic Acid Shiff reagent) for examination in the light microscope, or processed to be examined with electron microscope. PAS-positive cells were identified in the upper region of the shelves in the 13.5-day specimens. In 14.5-day embryos it was found that in areas where the reorientation process had begun, the PAS-positive cells extended a considerable distance into the forming shelves. When the shelves were examined electron microscopically, it was found that two types of mesenchymal cells were present. One type were cells containing many free ribosomes, whereas the second type were characterized by 1) presence of glycogen particles, 2) networks of microfilaments in the peripheral cytoplasm of the cell bodies, and 3) microfilaments in the pseudopodia. Areas of developing cartilage, skeletal, and smooth muscle and nerve ganglia were identified in the posterior region of the shelves in the 14.5-day specimens. The glycogen-containing cells had the same position distribution as the PAS-positive cells seen in the light microscope. It is suggested that these cells are derived from the cranial neural crest. It is also proposed that the migration of these neural crest cells into the palatal shelf might contribute to the production of the intrinsic shelf force essential for reorientation.