Lipophilicity of the injectable form of some antibiotics was measured with a micromethod. The antibiotic was dissolved in a small volume (1 ml) of phosphate buffer at a physiological pH (pH = 7.4) and was extracted by a small volume of octanol (1 ml). HPLC determinations of the antibiotic were performed in the two phases. log P ranged from +1.3 for chloramphenicol to -4.3 for ceftriaxone. A linear relationship was established for a few antibiotics between the log P values found in our experiments and the log permeability calculated from data in the literature for human and rat brain. This linear relationship enabled the brain concentrations of antibiotics to be predicted in man and rats.