The authors have proposed the evaluation of the possible pharmacokinetic interactions between xibornol and theophylline. This study has been carried out on eight healthy volunteers of both sexes between 19 and 27 years of age. On the first day each subject was treated with 600 mg oral sustained-release formulation of aminophylline at 08h00 and at 20h00. From the 2nd day an oral administration of 500 mg of xibornol every 8 h was added to the treatment of aminophylline. On the 1st, 2nd and 5th day blood samples were taken after 30, 90, 180 and 300 min following drug administration. After serum separation an analysis was performed on them. The results have shown that theophylline levels at the programmed times during the three days tested are just overlapping. In conclusion the administration of xibornol does not introduce any interference with theophylline clearance or lengthening of its plasma half-life.