Rat frontal bones reportedly exhibit lateral surface growth many weeks after fusion of the frontal suture. This is histologically confirmed in the present study. The mean width of the cranial vault at the postorbital constriction increased by 10 % between the age of 30 and 58 days. The increase must be ascribed to periosteal growth, as the lateral, external surfaces exhibited active growth. External apposition without concomittant internal resorption causes an increase in bone thickness. The cells on the internal surfaces were in a state of rest. The study was based on a limited material; therefore, definite conclusions cannot be drawn.