The combination of microdialysis and fast-atom bombardment mass spectrometry has been used to follow the pharmacokinetics of penicillin G directly in the blood-stream of a live rat. After the intramuscular injection of the antibiotic, the blood dialysate was allowed to flow into the mass spectrometer via the continuous-flow/fast-atom bombardment interface. Tandem mass spectrometry provided the means for isolating and recording the ion fragments produced from the drug as the dialysate was exposed to the ionization process. The pharmacokinetic results obtained are compared with those previously reported in the literature, and this on-line method is discussed in terms of its particular advantages.