The eyelid opening of pups born to and nursed by normal mice occurred by the 15th day of birth, whereas pregestational sialoadenectomy (removal of submandibular glands) of nursing mice delayed eyelid opening of their pups by as much as 5 days. Parotidectomy, however, had no effect on eyelid opening. Cross-foster nursing experiments indicated that the cause for delayed eyelid opening of pups was to be found in sialoadenectomized mothers, not their pups. Sialoadenectomized mothers had underdeveloped mammary glands that produced approximately 50% less milk than controls, and the amount of epidermal growth factor in their milk was similarly reduced. When epidermal growth factor, a polypeptide produced by the submandibular gland, was injected daily at a dose of 5 micrograms into sialoadenectomized pregnant mice, the eyelid opening of the pups nursed by their mothers occurred normally. These results are discussed with regard to the possible role of the submandibular gland and epidermal growth factor in neonatal eyelid opening.