In most Wuchereria bancrofti and Brugia malayi infections, the microfilaria are found in the blood in greatest number between 10 p.m. and 2 a.m., indicating that chronotherapy may be beneficial in treating such infections. This study reports the influence of time of administration on the pharmacokinetics of diethylcarbamazine (DEC) in healthy volunteers. The study was conducted in 12 healthy volunteers by administering a 150 mg single oral dose of diethylcarbamazine citrate at 0600 or 1800 h in a balanced crossover design with the approval of an institutional ethics committee. The subjects fasted for about 10 hours before and 3 hours after drug treatment. Blood samples were collected at predetermined time intervals, and the drug content in the serum was estimated using HPLC with an electrochemical detector. Pharmacokinetic analysis was performed using noncompartmental methods employing WinNonlin (version 3.1), and the means of various pharmacokinetic parameters were compared for any dosing time-related changes using a paired t-test at a probability level of 95%. The mean +/- SD values of pharmacokinetic parameters of DEC for the treatments at 0600 versus 1800 h were as follows: Cmax, 500+/-227 versus 637+/-401 ng/ml; tmax, 2.3+/-0.7 versus 2.7+/-1 h; Ka, 2.23+/-0.72 versus 1.96+/-0.97 h(-1); t1/2, 14.6+/-6.7 versus 11.4+/-4.9 h; AUC0-t, 5,334+/-1,853 versus 6,901+/-4,203 ng x h/ml; AUC0-infinity, 5,840+/-1,922 versus 7,220+/-4,205 ng x h/ml; CL/F, 36,058+/-19,011 versus 32,189+/-25,293 ml/h/kg; Vd/F, 570+/-225 versus 533+/-447 L; and MRT 17.7+/-5.9 versus 15.3+/-5.2 h. None of the parameters was significantly changed (p > 0.05) as a function of time of administration.