The objective was to develop a technique for calculating continuous, beat-to-beat aortic flow (AoF) using only left ventricular pressure (LVP) and aortic pressure (AoP). An electric analog model of the aortic valve was developed that includes resistance (R), inertance (L), and compliance (C) parameters, and resulting second order differential equations were derived. Aortic flow, AoP, and LVP recorded in eight subjects during a 5 day period and during lower body negative pressure (LBNP) were used to validate the model. Resistance, L, and C were estimated using a least-squares fit to the measured AoF on day 0 and during 0 mm Hg LBNP. For days 1–4, AoF was calculated using measured values of AoP and LVP and the R, L, and C values from day 0. Similarly, for LBNP, AoF was calculated using measured values of AoP and LVP, and the R, L, and C values from 0 mm Hg LBNP. The calculated and measured AoF were compared. Differences in cardiac output between the calculated and measured flows were less than 13.1 ± 17% across days and under minor altered physiologic conditions (LBNP). Waveform morphology for the calculated AoF also agreed well with the measured AoF. Spectral analysis showed differences in magnitude and phase between measured and calculated aortic flow for the first five harmonics across days, less than 20 ± 6% and 25 ± 14°, respectively. Preliminary evaluation indicates that our model works well for calculating flow through a biologic valve using LVP and AoP. We speculate that it may perform better for a mechanical valve, and if so it may be possible to develop an instrumented mechanical valve capable of continuous LVP, AOP, and AoF measurements. ASAIO Journal 1999; 45:204–210.