Studies in rats indicate that the metabolic activation of acetylhydrazine, a metabolite of isoniazid, is a critical determinant of the hepatotoxicity of isoniazid. As demonstrated in that model, the formation of 14CO2 after the administration of 14C-acetylisoniazid reflects the activity of the toxic pathway. A similar approach in man should make it possible to demonstrate the presence and to assess the quantitative importance of this toxifying pathway, and thus to evaluate its role in the pathogenesis of isoniazid hepatitis. We gave 300 mg isoniazid together with 10 microCi 14C-acetylisoniazid (12 mg) to 17 healthy subjects and determined the time course of the plasma concentrations of isoniazid, acetylisoniazid, acetylhydrazine, and diacetylhydrazine and of the exhalation of 14CO2. The time course of 14CO2 in breath closely paralleled the plasma concentration-time curve of acetylhydrazine but not those of acetylisoniazid or diacetylhydrazine, indicating that the 14CO2 originated directly from the metabolism of acetylhydrazine. The cumulative exhalation of 14CO2 increased with decreasing rate of acetylation of isoniazid, such that slow acetylators generated more 14CO2 than rapid acetylators. Simulation studies demonstrated that even if the data are corrected for the different formation of acetylisoniazid from isoniazid in slow and rapid acetylators, the slow acetylators still generated more 14CO2. The data therefore indicate that a substantial fraction of the acetylhydrazine formed from isoniazid passes through a pathway that has been shown in animals to generate highly reactive and hepatotoxic intermediates.(ABSTRACT TRUNCATED AT 250 WORDS)