The analysis of the new antimetabolite 2',2'-difluoro-2'-deoxycytidine (Gemcitabine, Eli Lilly Corp.) and of its metabolic deamination product, 2',2'-difluoro-2'-deoxyuridine, in urine and plasma by means of 19F NMR is described. Both compounds show AB-type NMR spectra which are characterized as a function of pH at a magnetic field strength of 9.4 T. In the physiological pH range the spectra of both compounds are distinct, despite the fact that both compounds have almost identical average 19F shifts. A linear relation between NMR intensity and concentration in urine has been established. The NMR method does not require sample pretreatment and allows for rapid, nondestructive analysis of Gemcitabine concentrations over 0.01 mM. Separation of the two compounds in vivo by 19F NMR spectroscopy is difficult to achieve.