Pneumonia was induced in a group of calves that were then treated with oxytetracycline (OTC). When the plasma pharmacokinetics and tissue concentrations of this group were compared with those of healthy controls, obvious differences were observed. The volume of distribution and the half-life of OTC were increased in the group with pneumonia. Tissue concentrations of OTC (1.48 +/- 0.29 microgram/ml) were greater in the lungs of calves with pneumonia than that in the lungs (1.09 +/- 0.09 microgram/ml) of healthy controls. For the dose of OTC used, the correlation between serum concentration and tissue concentrations at 24 hours was poor. Seemingly, current concepts of antibiotic dosages and dosage intervals may be inadequate because they are based on blood and tissue values of healthy animals.