The fate of orally administered ethionamide (2‐ethyl‐4‐thiocarbamoylpyridine) and ethionamide sulphoxide in man, rats, mice and dogs has been examined. Though spontaneous interconversion between the two antituberculosis drugs does not take place, there was extensive interconversionin vivo, both appearing in the blood within 15 min of dosing irrespective of which compound was administered. Species differences were evident in both the rate of metabolism and in the ethionamide: ethionamide sulphoxide ratio. Apart from this interconversion, ten probable urinary metabolites were detected. These consisted of six fluorescent and four non‐fluorescent metabolites, three of the latter being identified as 2‐ethylisonicotinamide, 2‐ethylisonicotinic acid and inorganic sulphate. A scheme is proposed to cover part of the common metabolic route of th