23Na NMR studies of large unilamellar vesicles of egg lecithin in salt solutions are reported. A shift reagent, the dysprosium nitrilotriacetate ion Dy[N(CH2CO2)3]3-2 has been used to distinguish between 23Na+ inside and outside the vesicles. When both are present and the shift reagent is present on only one side, two clearly distinct resonances are observed. Creation of a Na+ concentration gradient and subsequent catalysis of passive transport induced by the introduction of gramicidin can be monitored easily by using the relative intensities of the two resonances. We report the observation of transport both out of and into vesicles.