A method for in vivo determination of the concentration in a body fluid of metabolically significant substance comprises storing of a needle with a sensor in a liquid-filled ampoule, brief insertion of the needle into a patient, recording of the output signal from the sensor, withdrawal of the needle, and calculation of the concentration looked for on the basis of the recorded signals. The method is preformed using an apparatus comprising a needle (5) with a sensor (6), an ampoule (1) containing a solution (2) and closed by two membranes (3, 4). The needle (5) is stored with its sensor (6) in the solution. The apparatus further comprises a mechanical unit for driving the needle (5) to protrude from the ampoule for a set moment, and an electronic unit storing the output signal from the sensor (6) and performing calculations on the basis of the stored signal values. The needle with its sensor (6) and the ampoule with the solution (2) are independent disposable parts mounted in a durable part containing the mechanical and electronic parts.