In this Study, we use HRMAS NMR as a non-invasive technique to monitor the in vivo metabolism of a xenobiotic. The antituberculosis Ethionamide is a pro-drug that has to be activated in mycobacteria before inhibiting, its cellular target, The use of H-1 HRMAS NMR has allowed to detect a metabolite (ETH*) of the drug directly in living bacteria even with a spectrometer operating at the relatively low magnetic field of 300 MHz, We show that metabolism monitoring, of an unlabelled drug at a therapeutically relevant concentration as low, as 5 μ g/ml is within reach of the technique. II IIRMAS NMR in combination with diffusion filtering leads to the conclusion that the metabolite is located inside the intact cells. The comparison of the metabolite NMR Signature with that of synthetic Molecules proves the non-identity of ETH * with the ETH derivatives described previously in the literature. © 2005 Elsevier Inc. All rights reserved.