The effective duration of oxytetracycline 20 percent for injection in three different pharmacy formulae: short acting oxytetracycline injectable solution (OTC - 20) and two long acting (LA) oxytetracycline injectable solutions including OTC-VMD and OTC-PZR was studied on chickens. This study was based on quantifying antibiotic levels in the plasma of chickens using microbiological test for building up an appropriate therapy for this antibiotic. The plasma was collected from 3 experimental chicken groups after intramuscular injection with OTC solution. The results of testing plasma of the experimental chicken groups showed that at 24 hours after muscular injection, oxytetracycline levels in plasma of the chickens were similar in 3 experiments, these levels were: 1.56 ugIml
1.59 ugIml and 1.63 ugIml, respectively. At 48 hours after OTC-20 injection, the level of oxytetracycline decreased to 0.44 ug/ml, whereas at 48 and 72 hours, after OTC-VMD and OTC-PZR injection, oxytetracycline levels were still maintained at the concentrations of 1.20 - 1.02 ug/ml and 1.27 - 1.05 ug/ml respectively. The antibacterial activities of chicken's plasma after intramuscular injection of OTC showed that all of three oxytetracycline pharmacy formulae could inhibit and kill E.coli and Salmonella reaching the best result at 12 hours after injection. This capability was maintained up to 24 hours for the OTC-20, and up to 72 hours for OTC -VMD and OTC-PZR. There was no significant difference in the clinical expressions as well as histological injury at injected site in comparison between injection of chickens with two formulae, between local manufacture OTC and imported OTC. This result indicated that for effective therapy, the chicken should be injected daily with OTC-20 and every 3-days for OTC-VMD and OTC-PZR in an appropriate therapy.