The potential usefulness of in vivo 19F NMR spectroscopy for the monitoring of flomoxef sodium (FMOX) kinetics was evaluated. In the experimental study using phantom the minimum concentration of FMOX of which signals could be monitored with 19F NMR spectroscopy was 16 micrograms/ml. In patients with intravenous bolus injection and drip infusion of FMOX (2 g/100 ml), signals from normal heart, liver, and kidney were clearly monitored with 19F NMR spectroscopy. In normal lung any signal was not detected, but in a lung with a cancer associated massive atelectasis the monitoring of signals was possible with 19F NMR spectroscopy in both intravenous and intraarterial injections. Monitoring of FMOX kinetics with in vivo 19F NMR spectroscopy will be useful in clinical applications.