Macrocyclic lactones (ML) are highly effective anthelmintics that provide a long protective period after administration because of their extensive distribution into fat. This study examined whether the body composition of the animal at the time of treatment had any influence on the pharmacokinetics of two MLs, moxidectin (MOX) and ivermectin (IVM). 'Fat' and 'lean' lines of pigs were established using two different diets, with weekly determination of liveweight and backfat thickness confirming the difference in body condition between the groups. Blood samples were taken at regular intervals following i.v. injection of IVM or MOX at a dose of 300 microg/kg and the plasma was analysed using fluorescence high performance liquid chromatography (HPLC) to determine the concentration of IVM or MOX in the samples. Regardless of body composition IVM and MOX kinetics were very different with MOX having a greater apparent volume of distribution, longer distribution and elimination half-lives and a slower clearance rate than IVM, which led to MOX being detectable in plasma for >40 days compared with only 8-10 days for IVM. Altering body composition had no detectable influence on the kinetic disposition of IVM in this study. In contrast, although there was no difference in AUC or the volume of distribution, MOX was distributed within and eliminated from the lean animals more rapidly than from the fat animals.