OBJECTIVE The primary objective of this study was to characterize the pharmacokinetics and pharmacodynamics of single daily doses of donepezil (5 and 10 mg) each evening for 28 consecutive days. A secondary objective was to measure the plasma protein binding of donepezil at steady state. METHODS This was a double-blind, randomized, multiple-dose study in healthy male (n=13) and female (n=3) volunteers. Subjects were randomized to receive, once daily, either oral doses of 5 mg donepezil for 28 days or doses of 5 mg donepezil for 7 days followed by 10 mg donepezil for 21 days. All doses were administered in the evening. Donepezil concentrations and protein binding in plasma were determined by HPLC with UV detection and equilibrium dialysis, respectively. Inhibition of acetylcholinesterase (AChE) activity in red blood cell (rbc) membranes was assessed using a specific radioenzyme assay. RESULTS The pharmacokinetics of donepezil were linear, dose proportional and stationary over the course of the study. Mean Cmax, tmax, AUC(0-24), t1/2 and Vlambda(z)/F at steady state were 34.1 ng ml(-1), 3.0 h, 634.8 ng h ml(-1), 72.7 h, and 11.81 kg(-1), respectively, for the 5 mg group and 60.5 ng ml(-1), 3.9 h, 1127.8 ng h ml(-1), 73.5 h and 11.61 kg(-1), respectively, for the 10 mg group. Accumulation of the drug was observed for 14-21 days, until steady state was achieved. A direct consistent relationship was observed between donepezil plasma concentration and percentage rbc-AChE inhibition during each 24 h evaluation period, indicating no hysteresis in donepezil pharmacodynamics. The pharmacodynamic parameters, Emin, Emax and Ess, were 62.2%, 71.8% and 65.3%, respectively, for the 5 mg donepezil dose, and 74.7%, 83.6% and 77.8%, respectively, for the 10 mg donepezil dose. Donepezil was 95.6% bound to plasma protein at steady state. The binding was high capacity and low affinity, and neither concentration nor time dependent. Both dosage regimens were well tolerated; no clinically significant changes in laboratory or vital sign parameters were observed in any subject. CONCLUSIONS The measured pharmacokinetic and pharmacodynamic parameters for both 5 and 10 mg day(-1) donepezil administered in the evening are in good agreement with previous results obtained with morning administration, indicating no time of dosing effect.