A novel technique for in vivo intrabronchial pharmacokinetic measurements using microdialysis is described. The technique was applied to the measurement of tobramycin and gentamicin in the anesthetized rat. The concentration versus time profiles of the drugs in the lung epithelial lining fluid (ELF) following intravenous bolus administration were determined. The mean penetration ratios into the ELF were determined and found to be 0.36 +/- 0.06 (mean +/- SEM, n = 5) for gentamicin and 0.56 +/- 0.09 (mean +/- SEM, n = 5) for tobramycin. The findings suggest that the technique can be used for intrabronchial pharmacokinetic measurement of drugs in the lung, either as an extension of the bronchoalveolar lavage technique or as a practical alternative to it.