Abstract: Blood flow rate in the bloodstream of a live being is measured using glucose as an indicator continuously supplied to the bloodstream. A reference sample is taken in a selected of measured segment of the bloodstream and then, an indicator of known concentration and known flow is added to the bloodstream, while—with a time delay—there is performed the taking of at least one blood sample from a measuring point, located downstream, while the resulting flow rate through the measured segment is set on the basis of a functional relationship.