The concentrations of chloramphenicol and its water-soluble metabolites in the plasma of six clinically healthy heifers were measured at intervals during five days after intramuscular administration of free chloramphenicol (20 mg/kg) in a vehicle containing 40% of an organic solvent. Estimations were carried out by a colorimetric method and by high pressure liquid chromatography (for the very low values beyond the second day). For free chloramphenicol a peak concentration of 1.7 micrograms/ml at 7.3 h after injection was found (MIC: 5 micrograms/ml). Bioavailability was calculated to be 63%. It is shown that absorption was apparently not a uniform process but occurred rather slowly (t 1/2 (ab) = 10.2 h) for the main part of the available dose, whereas one sixth was quickly absorbed (t'1/2 (ab) = 0.7 h). The apparent half-time of elimination was 10.2 h for the unchanged drug. At the fifth day after administration the plasma concentration was below the limit of detectability (10 ng/ml) in all animals.