The bioavailability and pharmacokinetics of doxycycline hyclate were determined in calves with immature rumen function. The bioavailability of doxycycline after oral administration in a milk replacer was approximately 70%. The elimination half-life of doxycycline was found to be 9.5 +/- 3.0 h. after intravenous administration, and 12.6 +/- 5.0 h. after single oral administration. Plasma concentrations were determined after repeated oral administration of doxycycline dissolved in a milk replacer, at a dose of 5 mg per kg body weight, twice daily. During the period of administration, the plasma concentrations varied between Cmin of 1.0 +/- 0.19 mg/L and Cmax of 2.3 +/- 0.19 mg/L.