Aortic valve orifice area was dynamically measured in anesthetized dogs with a new measuring system involving electromagnetic induction. This system permits us real-time measurement of the valve orifice area in beating hearts in situ. The aortic valve was already open before aortic pressure started to increase without detectable antegrade aortic flow. Maximum opening area was achieved while flow was still accelerating at a mean of 20 to 35 msec before peak blood flow. Maximum opening area was affected by not only aortic blood flow but also aortic pressure, which produced aortic root expansion. The aortic valve orifice area's decreasing curve (corresponding to valve closure) was composed of two phases: the initial decrease and late decrease. The initial decrease in aortic valve orifice area was slower (4.1 cm2/sec) than the late decrease (28.5 cm2/sec). Aortic valve orifice area was reduced from maximum to 40% of maximum (in a triangular open position) during the initial slow closing. These measurements showed that (1) initial slow closure of the aortic valve is evoked by leaflet tension which is produced by the aortic root expansion (the leaflet tension tended to make the shape of the aortic orifice triangular) and (2) late rapid closure is induced by backflow of blood into the sinus of Valsalva. Thus, cusp expansion owing to intraaortic pressure plays an important role in the opening and closing of the aortic valve and aortic blood flow.