Moxidectin is a macrocyclic lactone drug derived from the actinomycete Streptomyces cyanogriseus and is currently being used as a veterinary product for the prevention of canine heartworm disease and for the treatment of internal and external parasites in cattle, sheep, goats, and horses. It is being developed by the World Health Organization (WHO) as a potential macrofilaricidal agent for mass drug administration for the elimination of onchocerciasis (river blindness) in humans caused by the parasitic worm Onchocerca volvulus. Ivermectin (Stromectol; Merck & Co., Inc.) is currently used in mass drug administration programs, but an alternative agent would potentially offer a choice in treatment. Mass drug administration is a process that would call for administration of single doses of medicine, under the supervision of a community health worker, to an entire community at one time.Knowing the extent of excretion of moxidectin into breast milk and the pharmacokinetics of the drug in lactating women will help the community health worker make appropriate decisions about dosing, hence the need for this study. Preclinical studies have shown that the absolute bioavailability of moxidectin is variable, ranging from 19% in rats to 90% in dogs (9). After oral administration, moxidectin is quickly absorbed, with the time to peak plasma concentration (t max ) being 3.7 Ϯ 1.5 h in 27 fasting healthy volunteers receiving an 8-mg dose. Administration of moxidectin with food has been shown to increase the mean peak plasma concentration (C max ) and total area under the concentration-time curve (AUC) by 34% and 39%, respectively (8).The apparent volume of distribution (V z /F) of moxidectin is large (2,000 to 3,500 liters), and moxidectin has a low clearance (CL) (2.37 to 3.50 liters/h) and undergoes very slow elimination, with the terminal-phase elimination half-life (t 1/2 ) being 485 to 842 h (4).A study in lactating dairy sheep showed that after administration of a single oral dose of 200 g/kg of body weight of moxidectin, 2.1% Ϯ 0.33% of the administered dose was excreted in milk for more than 35 days after dosing (7). The ratio of the total AUC curve for moxidectin in breast milk (AUC milk ) to the AUC was 14.3 Ϯ 1.88, indicating that although the total amount of drug that entered the breast milk was small, moxidectin preferentially entered breast milk compared to results for plasma. Similar results were seen in a study in goats (1), where 5.7% Ϯ 1.0% of an orally administered dose of 0.2