The aim of this study was to investigate pharmacokinetics and penetration into the aqueous humor of Long Action Oxytetracycline (OTC-LA) after Intravenous (IV) and Intramuscular (IM) administrations, at a single dose of 20 mg kg(-1) b.wt in rabbits. After administrations of the long action formulation, the plasma oxytetracycline concentrations were evaluated using a two-compartmental open model. The study was designed according to a two-period cross-over and the plasma OTC concentration was measured using the ELISA procedure. The elimination half-lives (t(1/2 beta)) of the OTC-LA after IV and IM administrations were 12.60 +/- 0.92 and 38.67 +/- 4.40 h, respectively. After IV administration, the volume of distribution (V(dss)) and total body Clearance (Cl(tot)) values of the drug were 3.42 +/- 0.21 L kg(-1) and 0.19 +/- 0.01 L/h/kg, respectively. The maximum concentration of the drug (C(max)) in the plasma (4.23 +/- 0.43 mu g mL(-1)) was achieved at 2.0 h (t(max)) after IM administration. The Minimum therapeutic plasma Concentration (MIC) of the drug at the amount >= 0.5 mu g mL(-1) was maintained until 48 h after IV and IM administrations. The intramuscular bioavailability of the drug was 0.79 +/- 0.10%. After IV and IM administrations of OTC-LA formulation, the maximum concentrations of the drug in the aqueous humor were 0.1 and 0.068 mu g mL(-1), respectively. However, the concentrations of the drug in the aqueous humor were below the MIC value (0.5 mu g mL(-1)) during 12-72 and 4-48 h for IM and IV administrations, respectively.