1. Metabolism of the antibacterial, piromidic acid (5,8-dihydro-8-ethyl-5-oxo-2-pyrrolidinopyrido[2,3-d]pyrimidine-6-carboxylic acid) was investigated in rats and human subjects. Ten metabolites and the unchanged drug were found in the urine and the bile of both species after oral administration.2. Metabolites were identified by comparison with authentic materials, except for the unstable metabolite, M-VI, for which a probable structure is proposed. The metabolic pathway of piromidic acid involved hydroxylation in the pyrrolidine ring to give the 2- and 3-hydroxy-derivatives (M-II and M-V). M-II was further metabolized to the correspondingγ-aminobutyric acid derivative (M-IV) and the 2-5-dihydroxypyrrolidine derivative (M-VI) which was further metabolized to the 2-amino-pyridopyrimidine carboxylic acid (M-III). Piromidic acid, M-V, M-II, M-III and M-IV were partly excreted as respective glucuronides.3. Metabolites, except glucuronides, exhibited antibacterial activity; M-V and M-II showed greater activity than piromidic acid.4. The metabolism of piromidic acid is discussed in relation to the physico-chemical properties of the drug and its metabolites.