Portapres® derives continuous estimates of cardiac output from the peripheral pulse and has the potential to be an extremely valuable physiological and clinical tool. We assessed Portapres® estimates of cardiac output in healthy subjects at rest, during maximal treadmill exercise (n=8) and during decreases caused by orthostatic stress (n=8). Comparison with a rebreathing method indicated that Portapres® tended to overestimate cardiac output. The random errors of the estimates (precision), expressed as ±2 S.D. of the differences between paired estimates during steady states, ranged between 1.2 and 2.6 litres/min. We conclude that these errors indicate that the method is probably only useful for assessing changes in individual subjects where large changes are anticipated, as during exercise. When smaller changes occur, as during orthostasis, the errors preclude the use of individual subject data and only permit group average data to be examined.