A method for determining the liver performance of a living organism, in particular a human, comprising administering at least one 13C labelled substrate, which is converted by the liver by releasing at least one 13C labelled metabolization product, and determining the amount of the at least one 13C labelled metabolization product in the exhalation air over a definite time interval by the means of at least one measuring device with at least one evaluation unit is disclosed. Using this method, it is possible to describe the measured initial increase of the amount of the at least one 13C labelled metabolization product in the exhalation air using a differential equation of first order and to determine a value Amax (DOBmax) and a time constant tau of the increase of the amount of 13C labelled metabolization product from the solution of the differential equation of first order.
