A sensor assembly for an in vivo physiological pressure measurement has a member with a distal end to ease placement of the catheter sensor assembly in vivo. A passage is preferably in an intermediate part of the member and is open but covered by a pressure responsive sensor sealing the passage. The sensor has a first side away from the intermediate part for exposure to in vivo physiological pressure and a second side in fluid communication with the passage. A pressure connecting means for inhibiting liquid communication located between the physiological pressure and the passage is arranged for selectively connection therebetween so that when connected the pressure on the first and second sides of the sensor can be substantially equalized. A control ex vivo makes the selective connection of the second side of the pressure sensor and the physiological pressure. A method for rezeroing an in vivo pressure sensor has the steps of obtaining a signal from the first side of the sensor at distal end, using a pressure substantially equal to the in vivo pressure and applying that pressure to the second side of the sensor.