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The work reported here describes the comparative ability of albendazole (ABZ), fenbendazole (FBZ) and triclabendazole (TCBZ) to penetrate through the tegument of mature Fasciola hepatica, and the influence of the physicochemical composition of the incubation medium on the drug diffusion process. The data obtained from the trans-tegumental diffusion kinetic studies were complemented with the determination of lipid-to-water partition coefficients (octanol-water) for the benzimidazole (BZD) anthelmintic drugs assayed. Sixteen-week-old F. hepatica were obtained from untreated artificially infected sheep. The flukes were incubated (37 °C) over 60 and 90 min in incubation media (pH 7·4) prepared with different proportions of ovine bile and Krebs' Ringer Tris (KRT) buffer (100, 75, 50, 25 and 0% of bile) containing either ABZ, FBZ or TCBZ at a final concentration of 5 nmol/ml. After the incubation time expired, the liver fluke material was chemically processed and analysed by high performance liquid chromatography (HPLC) to measure drug concentrations within the parasite. Additionally, the octanol-water partition coefficients (PC) for each molecule were calculated (as an indicator of drug lipophilicity) using reversed phase HPLC. The 3 BZD molecules were recovered from F. hepatica at both incubation times in all incubation media assayed. The trans-tegumental diffusion of the most lipophilic molecules ABZ and FBZ (higher PC values) tended to be greater than that observed for TCBZ. Interestingly, the uptake of ABZ by the liver flukes was significantly greater than that measured for TCBZ, the most widely used flukicidal BZD compound. This differential uptake pattern may be a relevant issue to be considered to deal with TCBZ-resistant flukes. Drug concentrations measured within the parasite were lower in the incubations containing the highest bile proportions. The highest total availabilities of the 3 compounds were obtained in liver flukes incubated in the absence of bile. Altogether, these findings demonstrated that the entry of the drug into a target parasite may not only depend on a concentration gradient, the lipophilicity of the molecule and absorption surface, but also on the physicochemical composition of the parasite's surrounding environment.