Diethylcarbamazine (DEC) is the first-line drug for the control and treatment of lymphatic filariasis caused by the Wuchereria bancrofti and Brugia malayi parasite. DEC is rapidly and extensively metabolised in the liver. Rifampicin and ketoconazole are a potent inducer and inhibitor of hepatic cytochrome P450 enzymes (CYPs) resulting in numerous clinically significant drug interactions. The aim of this study was to examine the effects of co-administration of rifampicin and ketoconazole on the pharmacokinetic parameters of DEC in 12 healthy Thai male volunteers in an open-labeled, randomised 3-phase crossover design experiment. In phase 1 each volunteer received only a single oral dose of 6 mg/kg of DEC. In phases 2 and 3 the subjects received a single oral dose of 6 mg/kg DEC after pretreatment with either 600 mg rifampicin or 400 mg ketoconazole orally for 5 days, respectively. Each phase of each study was separated by a 1-month separation period. The plasma DEC concentrations during a 48 hour period were measured using High Performance Liquid Chromatography (HPLC). Statistical analysis using two-way ANOVA indicated that neither rifampicin nor ketoconazole significantly altered the mean Cmax, AUC0-48, AUC0−∝, t1/2, tmax, ka, ke, Vd/F and Cl/F ( P  >  0.05). There were no significant differences among the mean urine pH values of the 3 phases ( P > 0.05). The results show that the pretreatment with rifampicin and ketoconazole did not significantly affect any of the pharmacokinetic parameters. In this study, it was not possible to conclude clearly that DEC was not metabolized via the cytochrome P450 enzymes.