Real-time in-cell (FNMR)-F-19 spectroscopy is developed to determine the time scale of cellular uptake of fluorescent and nonfluorescent F-19-octaarginines. The peptide concentration as low as 10-mu M order is successfully detected with a time resolution of 1-2min. The concentration analysis shows that fluorescein isothiocyanate (FITC) induces different kinetics for the peptide uptake into natural human cells. The ability of (FNMR)-F-19 to detect the real-time uptake of bioactive molecules is shown with minimal perturbation.