This study investigated the effects of the concomitant administration of theophylline and toborinone on the pharmacokinetics of both compounds in poor and extensive metabolizers via CYP2D6. In period 1, a single dose of 3.5 mg/kg theophylline was administered orally. In period 2, a single dose of 1.0 microg/kg/min toborinone was infused over 6 hours. In period 3, 3.5 mg/kg theophylline was coadministered with 1.0 microg/kg/min toborinone. Serial blood and pooled urine samples were collected before and after toborinone administration for the quantification of toborinone and its metabolites in plasma and urine. Serial blood samples were collected before and after theophylline administration for the quantification of theophylline and its metabolites in plasma. No significant differences were observed in toborinone pharmacokinetics between poor and extensive metabolizers via CYP2D6. Toborinone coadministration with theophylline did not result in a substantive effect on the disposition of theophylline and vice versa.