Serum concentrations of chlortetracycline (CTC) in healthy chickens were determined for 24 h after administration, (with and without citric acid) as an oral (25 mg/kg) or i.v. (0.9 mg/kg) dose. The oral time-course drug data were fitted adequately by a 2-compartment pharmacokinetic model with absorption. The resulting absorption rate constant for the birds orally given CTC with citric acid was nearly equal to that for the birds given CTC alone. Although the uptake of orally administered CTC was rapid, only a small fraction of the dose was absorbed. The administration of citric acid-CTC significantly increased the mean serum concentration of CTC and the fraction of the dose absorbed. The citric acid-CTC mixture also produced significantly higher elimination and distribution rate constants for CTC.