A considerable body of research has been carried out in order to throw light on the pharmacological and toxicological impact of ATP-binding cassette (ABC) drug efflux transporters such as P-glycoprotein and Breast Cancer Resistance Protein (BCRP/ABCG2/MXR). Most studies focus on their role in rendering cancer cells resistant to anticancer drugs. Drug transporters are expressed in many tissues and they are strongly involved in the oral bioavailability, and the hepatobiliary, direct intestinal and renal excretion of many drugs. In veterinary therapy, some anti parasitic drugs and/or their metabolites, such as ivermectin, moxidectin, albendazole sulfoxide, which are widely used, have been shown to be actively transported by efflux pumps. This interaction plays an important role in drug disposition since its inhibition has been shown to increase the drug bioavailability in some domestic species. Moreover, some authors have reported that parasite resistance to anthelmintic drugs may be mediated by parasite P-glycoprotein efflux. In addition, the importance of milk residues for human nutrition has aroused increasing concern about the inadvertent transfer of drugs and other substances into mammary milk of domestic animals, potentially posing a health risk to consumers. Recently, the important role of BCRP in the secretion of its substrates in milk has been demonstrated.