Stroke volume (SV) estimation from computer analysis of aortic pressure contour was tested in 12 anesthetized dogs and 2 humans with A-V block. The ‘actual’ SV was calculated from right-heart bypass pump flow values in 6 open-chest dogs, from heart rate (HR) and cardiac output (CO) using dye dilution in 6 closed-chest dogs and by the Fick method in the human subjects. SV changes were induced by changes in pacing rate and pump flow, infusion and withdrawal of fluids and exercise. Mean arterial pressure was altered in dogs by changes in CO and infusions of drugs.In the dogs, the method (using Warner’s formula) proved consistent (r = 0.774, p < 0.001, S<sub>y. x</sub> = 3.82 ml) but inaccurate (slope = 0.386) in 627 comparisons. The same conclusions were reached in 11 human comparisons. The Kouchoukos formula showed similar problems. Corrections are suggested based on these results.