Abstract: The invention relates to a lid element which is placed on cell culture vessels in which cells contained in a liquid medium are accommodated. The lid element according to the invention is intended to allow determination of metabolism activities of cells which are contained in the cell culture vessels, by means of optical measurement methods, which can be carried out with simple handling by means of laboratory personnel. Light-guiding elements are provided on the lid element which can be fitted to the cell culture vessel and, when the lid element is fitted, project into the interior of cavities in the cell vessel. At least one optically sensitive layer is formed on one end surface and/or on the outer envelope surface of the light-guiding elements, which are preferably optical waveguides in the form of rods, for detection of chemical substance concentrations which change within the cavities.