The invention refers to a method and an apparatus which in situ measure the flow in a blood vessel mean (9). A local disturbance of the optical properties of the blood is performed at the measuring place. The disturbance is optically detected by means of an optical conduit (1) introduced into the vessel, the field of view of the conduit being directed towards the central portion of the vessel. The detected optical signal is used to form a measure of the flow. The device comprises a conduit introduced into the vessel up to the measuring place. A fibre optical light conductor (7) is provided within the conduit. According to the invention the end surface of the light conductor is bevelled to look into the central portion of the blood vessel. A light detector (7) is provided in the proximal end of the light conductor and detects the local disturbance of the optical properties of the blood. The disturbance may be brought about by introducing a fluorescent substance into the blood or by making use of the autofluorescence of the blood. Disturbances may also be brought about by locally producing microbubbles in the blood upstreams of the measuring position or by locally disturbing the orientation of the blood corpuscles.