Abstract: The method of noninvasively measuring concentration of a dye in a circulatory system includes measuring an optical density of an arterial blood-dye mixture in an artery as a function of time at a wavelength .GAMMA..sub.1 and at another different wavelength .GAMMA..sub.2 ; calculating ratios of optical density at the wavelength .GAMMA..sub.1 to that at wavelength .GAMMA..sub.2 of arterial blood in the artery at a time before a predetermined initial time and of the arterial blood-dye mixture at at least one time t after the predetermined initial time; injecting a dye absorbing electromagnetic radiation at at least one of the wavelengths into the circulatory system at the predetermined initial time, t=0, to form the arterial blood-dye mixture; measuring a hemoglobin concentration in a blood sample from the living organism; and calculating a concentration C(d,t) of the dye in the arterial blood-dye mixture as a function of time, t, from the hemoglobin concentration, Hb; from extinction coefficients, .epsilon..