An on-line dialysis solid-phase extraction gas chromatographic (GC) approach has been developed for the determination of drugs in plasma, using some benzodiazepines as model compounds. Clean-up is based on performing the dialysis of 100 μl samples for 7 min using water as acceptor phase and trapping the diffused analytes on a PLRP-S copolymer precolumn. After drying of the precolumn with nitrogen for 15 min, the analytes are desorbed with ethyl acetate (275 μl) and injected on-line into the GC system via a loop- type interface. The system provides a very efficient clean-up, and offers the possibility of adding chemical agents which can help to reduce drug protein binding and, thus, increase sensitivity. To demonstrate the potential of the described approach, the determination of benzodiazepines in plasma at their therapeutical levels is used as an example with flame ionization, thermionic and mass-selective detection.