SUMMARYDoxycycline was given to two groups of eight chickens at a dose of 20 mg/kg of body weight, intravenously (i.v.) or orally. Plasma concentration was monitored serially for 12 h after each administration. Another group of 30 chickens was given 20 mg/kg orally every 24 h for 4 days, and plasma and tissue concentrations determined serially after the last administration. Concentrations of doxycycline were measured using high-performance liquid chromatography. Pharmacokinetic variables were calculated, using a two-compartment open model. The elimination half-life and the mean residence time for plasma were 6.03 ± 0.45 and 7.48 ± 0.38 h, respectively, after oral administration and 4.75 ±0.21 and 2.87 ±0.11 h, respectively, after i.v. administration. After single oral administration, doxycycline was absorbed rapidly, with T max of 0.35 ± 0.02 h. Maximum plasma concentration was 54.58 ± 2.44 μ/ml. Oral bioavailability of doxycycline was found to be 41.33 ± 2.02%. Doxycycline was widely distributed in tissues and considerable concentrations were found following oral administration of 20 mg/kg on four successive days. The results indicate that doxycycline concentrations were cleared slowly and were at or below the accepted drug tolerance levels in the marker tissues within 5 days after dosing.

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