OBJECTIVE The aim of the study was to evaluate the pharmacokinetics of theophylline administered alone, and in combination with donepezil HCl, following multiple-dose administration of both drugs in healthy volunteers. METHODS This was an open-label, randomized, two-period crossover study in healthy male volunteers (n=12). During each treatment period, subjects received either titrated-dose theophylline alone, or in combination with donepezil (5 mg, once daily) for 10 consecutive days. On day 10 of each treatment period, serial blood samples for the determination of theophylline concentrations in plasma were measured up to 24 h. Treatment periods were separated by a 3-week, drug-free washout. Plasma concentrations of theophylline were determined by HPLC with UV detection. RESULTS No statistically significant differences in theophylline pharmacokinetics (Cmax, AUC or tmax) were observed between theophylline administered alone and in combination with donepezil. No clinically significant changes in vital signs, ECG parameters or clinical laboratory tests were observed in any subject during any treatment period. CONCLUSIONS Concurrent administration of donepezil HCl does not alter the pharmacokinetic profile of theophylline following multiple-dose administration of both drugs in healthy volunteers. These findings suggest that donepezil may be safely co-administered with theophylline without a need for dose modification or additional monitoring procedures.