ABSTRACT Despite its toxicity and low efficacy in the chronic phase, benznidazole is the drug of choice in Chagas disease. Scarce information about pharmacokinetics and pharmacodynamics of benznidazole has been published. We performed a phase I, open-label, nonrandomized pharmacokinetic study of benznidazole (Abarax) conducted with 8 healthy adult volunteers at the Infectious Diseases Department of the Vall d'Hebron University Hospital (Barcelona, Spain). The separation and detection of benznidazole were performed on a Waters Acquity ultraperformance liquid chromatography system (UPLC) coupled with a Waters Xevo TQ MS triple quadrupole mass spectrometer. The pharmacokinetic parameters were calculated based on a noncompartmental body model using Phoenix WinNonlin version 6.3 software. Furthermore, computational simulations were calculated for the multiple-dose administration at two dose regimens: 100 mg of benznidazole administered every 8 h and 150 mg of benznidazole administered every 12 h. After benznidazole administration, the median area under the concentration-time curve from time zero to time t (AUC _{0– t} ) and extrapolated to infinity (AUC _0–∞ ) were about 46.4 μg · h/ml and 48.4 μg · h/ml, respectively. Plasma benznidazole concentrations peaked at 3.5 h, with maximal concentrations of 2.2 μg/ml, and benznidazole exhibited a terminal half-life of 12.1 h. The median maximum concentration ( C _max ) of benznidazole was lower in men than in women (1.6 versus 2.9 μg/ml), and median volume of distribution ( V ) as a function of bioavailability ( F ) was higher in men than in women (125.9 versus 88.6 liters). In conclusion, dose regimens (150 mg/12 h or 100 mg/8 h) reached a steady-state range concentration above of the minimum experimental therapeutic dose. Sex differences in the benznidazole pharmacokinetics were observed; mainly, men had lower C _max and higher V / F than women.