Recently it was reported that the use of the hydroxarnate-inhibicing method leads to an underestimation of the in vivo alternative pathway (AP) activity and its contribution to total respiration (V t ) in plants. However, there is still a dispute about the extent of these possible underestimations. In this paper the oxygen-isotope-discrimination (OID) technique and the hydroxamate-inhibiting method have been compared for aged potato (Solanum tuberosum L.) tuber slices. As a result, the in vivo contributions of AP activity to V t , were calculated to be 15.7% and 16.8% with the hydroxamate-inhibiting method, and 30.2% and 34.9% with the OID method in slices aged for 12 and 24 h, respectively. The observed underestimations of about one-fold confirm the suggestions with respect to the unsuitability of the hydroxamate-inhibiting method for the determination of the in vivo AP activity.