Plasma concentration-time data obtained after an oral single dose of 500-mg valproic acid (VPA) in a delayed release formulation was used to model enterohepatic cycling of the drug. Fourteen healthy subjects (seven women and seven men) were enrolled, food intake was standardized, blood samples were withdrawn up to 48 h post dosing and VPA plasma concentrations were analyzed by HPLC-UV method. Secondary peaks of VPA were observed in almost all subjects. A population pharmacokinetic analysis with sex, weight, age and contraceptive therapy as possible covariates was performed. Women without contraceptive therapy presented a longer lag time, a lower VPA hepatic output and a higher reabsorbed fraction than men. Weight affected both volume of the central compartment and the absorption rate constant. Women under contraceptive therapy showed similar disposition parameters to men. Reabsorbed fraction of bioavailable dose was 46.2 % in women and 21.8 % in men, revealing important sex differences in drug hepatobiliar output. These results showed that VPA displays sex-related pharmacokinetic differences due to different metabolic and transporter-mediated disposition.