Knowledge of the intracranial pressure would prove medically valuable in the treatment of congenital hydrocephalus and severe head trauma. An indwelling sensor, utilizing a radioactive material as the pressure transmission signal has been developed. The unit is compact, totally implantable, requires no electrical power, and is self-compensating for external temperature and pressure. Development of the sensor has proceeded through in vitro and primate in vivo testing. Clinical trials are expected in early 1974.