A single dose kinetics of isoniazid and rifampicin alone, and combination was studied in well-nourished and malnourished patients of tuberculosis. The elimination half-life of isoniazid was significantly increased when administered in combination with rifampicin in well-nourished and malnourished patients, while no significant difference was observed in any of the pharmacokinetic parameters of rifampicin in combination with isoniazid or alone in both groups of patients. Results are discussed on the basis of pharmacokinetic drug interaction.