1. High-resolution 1H nuclear magnetic resonance (1H-n.m.r.) spectroscopy at 300 MHz has been used to monitor the excretion in human urine of the drug metronidazole and its metabolites. 2. One or more resonances from all of the major metabolites are resolved in spectra of whole urine, and can be used to quantify excretion of the parent drug and its metabolites. The spectra also contain resonances from a large number of endogenous metabolites, many of which are readily identified. 3. 1H n.m.r. should be useful in monitoring, simultaneously and non-destructively, the urinary excretion of drugs and drug metabolites, as well as endogenous compounds in either normal or disease states.