1. The metabolism of [2,6-14C]pyridine in vivo has been investigated in the rat, hamster, mouse, gerbil, rabbit, guinea-pig, cat and man, and the quantitative determination of the various urinary metabolites carried out by radiochromatographic analysis. 2. Unchanged pyridine and its N-methylated metabolite, N-methylpyridinium ion, were determined using a Partisil-10 SCX cation-exchange h.p.l.c. column, whereas the C- and N-oxidation products were assayed by reverse-phase chromatography, using a Partisil-10 ODS column. 3. Most of the species studied produce pyridine N-oxide, N-methylpyridinium ion, 2-pyridone, 3-hydroxypyridine and 4-pyridone as metabolites, but the proportion of the dose excreted as each of these metabolites is species-dependent.