A method for measuring in non-invasive manner the concentration of blood constituents by which backscattered light is measured under the action of ultrasonic radiation focused towards the inside of a central blood vessel. A light source and detection unit are arranged to detect the backscattered light on the skin surface above the blood vessel. The target tissue is illuminated by two discrete optical wavelengths. An average light intensity distribution is detected over the length of a pulse. The distribution is Fournier transformed, and the largest Fournier components and spectral position are determined in relation to the frequency of the ultrasonic radiation. The component concentration in the blood vessel is calculated taking into account the volume of the ultrasonic focus contributing to the signal and blood flow rate.