Using pneumo-encephalographic measurements the author describes the ratio A: B method for determination of the normal position of the cerebral aqueduct in the age group 0 to 22 month, based on the measured distance from skull surface to aqueduct (A) and that from the skull surface to dorsum sellae (B). Normal value of the ratio A: B was 0.72±0.02. Compared with earlier investigations on the same topic in the age group 2 to 62 years, identical normal values were calculated. Thus the A: B method is independent of the age and the skull dimensions.