The invention relates to a method of the noninvasive optical in-vivo measurement of properties of flowing blood in a blood vessel within a body, for example for determining the concentration of blood constituents, wherein the body is irradiated with ultrasound radiation at an ultrasound frequency (fUS) in order to label a blood vessel, the body with the blood vessel is illuminated with light with at least one light wavelength and the back-scattered light is detected with a detector, the light component backscattered by the body outside of the blood vessel is modulated by a frequency (fMG) that corresponds to the frequency (fUS) of the ultrasound radiation, and the light component backscattered inside the blood vessel is modulated due to the Doppler effect in flowing blood with a frequency (fΜΒ) that is shifted by the Doppler shift (fD) with respect to the frequency (fUS) of the ultrasound radiation, and an evaluation device extracts the signal component modulated by the shifted frequency (fΜΒ) from the detector signal measured at the detector. Fig. 1
