The metabolism and futile deacetylation of phenacetin has been investigated in the rat via 1H NMR spectroscopic analysis of urine. Animals were dosed with either phenacetin or phenacetin-C2H3 and urine samples were collected for -24-0 (pre-dosing), 0-8. 8-24, and 24-48 h post-dosing. Drug metabolites of the two compounds were concentrated from the urine using solid-phase extraction prior to the use of directly-coupled HPLC-1H NMR spectroscopy for separation and identification. Following dosing of phenacetin, the metabolites identified were paracetamol glucuronide, paracetamol and N-hydroxyparacetamol, whilst paracetamol and N-hydroxyparacetamol sulphate were identified following dosing of phenacetin-C2H3. Quantitatively the percentage futile deacetylation of phenacetin-C2H3 metabolites was found to be 32% in both paracetamol and N-hydroxyparacetamol sulphate. This study further indicated the importance of futile deacetylation in simple analgesics and the value of directly-coupled HPLC-NMR spectroscopy for the study of this process.