Abstract: A method and apparatus for zeroing and calibrating an invasive blood pressure monitoring system includes a method whereby the diaphragm of a strain gauge type blood pressure transducer may be subjected to a zero reference pressure for zeroing and an elevated pressure for calibration of a blood pressure monitoring system in order that an oscilloscope display of the blood pressure waveform of the patient may be accurately monitored by visual inspection of an oscilloscope screen. During zeroing and calibration of the blood pressure monitoring system, the invasive interconnection with the bloodstream of the patient is ensured to be closed to prevent the occurrence of air embolus and the blood pressure transducer system is also isolated from the atmosphere at all times to prevent invasion of the system by means of bacteria.