Different procedures have been since 1965 by the authors to test with isotopes the patency of shunts for hydrocephalus. The authors now measure the time 99m Technetium takes to appear in the thyroid gland after injection in the flushing device. This method shows easily and very quickly, with no computing, the dynamics of C.S.F. shunts. No hospitalisation is necessary and it is often possible to determine the cause of the disturbance in the shunt.