A study was done to investigate the effect of parasitism on patterns of doramectin (DRM) fecal elimination in lambs. Fourteen Suffolk Down parasitized lambs (26.9±1.5kg body weight: bw) were purposely selected for the study. Seven pairs of lambs were allocated into two experimental groups. Group I (non-parasitized) was pre-treated with 3 repeated administrations of 5mg/kgbw of fenbendazole to maintain a non-parasitized condition. In Group II (parasitized), the lambs did not receive any anthelmintic treatment. After 85d of the pre-treatment period, both groups were treated with a subcutaneous injection of 200γg/kgbw of DRM. Fecal samples were collected at different times between -85d before and 60d after the DRM treatment, for both parasitological and chromatographic analysis. Samples were analyzed by high-performance liquid chromatography (HPLC) with fluorescence detection. Data of DRM concentrations were expressed as wet weight. A non-linear pharmacokinetic analysis was performed and results were compared using the Mann Whitney test. Fecal maximum concentrations (Cmax) of DRM were 1.37±0.19γg/g (parasitized group) and 0.86±0.15γg/g (non-parasitized group) observed at the time of the maximum concentration (Tmax) of 2.1±0.4 and 3.1±0.3d, respectively. Differences in Cmax values were significant (P<0.05). The accumulated elimination of DRM in feces, expressed as the percentage of DRM total dose, was 67.1% in the parasitized group, whereas in the non-parasitized group it was 56.5%. Our results showed that gastrointestinal parasitic diseases can modify the patterns of DRM fecal elimination, when the drug is administered by subcutaneous route in lambs. © 2010 Elsevier Inc.