In order to assess the effect of flow on peak transmitral velocity and pressure half time and to test the Hatle orifice area formula, four Carpentier-Edwards mitral prostheses were studied in a positive displacement flow simulator at 20 different stroke volume/rate combinations using a constant left ventricular filling curve. Peak transmitral velocity was directly related to mean flow and differed significantly between the individual values (F79 = 38; p less than 0.00001). There was, however, no significant difference in pressure half time between the four values (F79 = 0.04; p = 0.99). Pressure half time was directly related to diastolic time interval (r = 0.98, y = 0.41x - 17.84; p less than 0.00001), but was independent of stroke volume. There was only a moderate inverse correlation between pressure half time (T1/2) and orifice area (r = 0.56, y = 0.67 + 69/T1/2; p less than 0.0001). These results are consistent with the suggestion that pressure half time is more closely related to the shape of the left ventricular filling curve than to orifice area in normally functioning bioprostheses.