Ticlopidine, a new antithrombotic agent, and theophylline, a widely used bronchodilator drug, are both almost completely metabolized in the liver. To evaluate an interaction between these two drugs, we studied theophylline pharmacokinetics before, after 10 days of ticlopidine administration, and 1 month later in 10 healthy volunteers. We found a highly significant increase in the theophylline elimination half-life (P less than 0.001) and a comparable reduction in its total plasma clearance (P less than 0.001) after ticlopidine treatment. Pharmacokinetic parameters returned to initial values within 30 days after ticlopidine withdrawal. Moreover, no changes in theophylline pharmacokinetic parameters were observed 3 months later, before and after 10 days of placebo administration. Our results seem to exclude direct liver toxicity and may suggest a reversible inhibition of the liver metabolic capacity of theophylline.