Abstract: The plasma pharmacokinetics of doramectin (DRM) after intravenous (i.v.) and intramuscular (i.m.) administration at a single dose of 300 μg/kg b.w. were determinined over a 45-day period in pigs. Ten crossbreed pigs, clinical healthy, parasite free, weighing 36~50 kg were selected for study. Pigs were located into 2 groups of 5 animals and were treated with 300 μg/kg b.w. DRM with i.v. and i.m. injection, respectively. Blood samples were collected from jugular vein at different time points in 45 days after drug administration. The separated plasma were derivatized after solid phase extraction and analysed by high performance liquid chromatography (HPLC). Pharmacokinetics parameters were calculated by a computerized kinetic program MCPKP. The mean plasma concentration of DRM after i.v. and i.m. administration were detected until 30 and 25 days, respectively. Both drug concentration-time data of DRM with i.v. and i.m. administration accorded with two compartment open model in pig. The main pharmacokinetics parameters were: T_1/2α (1.092±0.66) d，T_1/2β (6.11±2.73) d, AUC （274.19±119.89） μg ·L-1·d-1 for i.v., and T_1/2α (0.056±0.022) d，T_1/2β (3.20±1.34) d，AUC (223.51±65.20) μg·L-1·d-1， C_MAX (28.99±10.69) μg/L，Tp (1.24±0.87) d for i.m.，respectively. The general bioavailability of DRM with i.m. injection was 81.5%. The results indicated that DRM was rapidly absorbed and distributed, slowly eliminated and possessed large volume of distribution, high bioavailability in pig with i.v. and i.m. administration. It suggested that the long activity of DRM in pig was likely to resulted from the intrinsic characteristic of DRM rather than different routes of drug administration or the kind of solvent used. Therefore, the results are helpful for understanding the pharmacokinetics characteristics and correct using of DRM in pig in clinic.