We have applied a nuclear magnetic resonance (NMR) method, based on the 7Li nucleus, to discriminate between intracellular and extracellular lithium ions (Li+) in red blood cell (RBC) suspensions. The NMR method was compared with atomic absorption, a technique that requires physical separation of intra- and extracellular Li+ prior to chemical analysis. The rates and rate constants of RBC Na(+)-Li+ countertransport measured by the 7Li NMR method correlated significantly with the measurements made by atomic absorption for both the hypertensive (r = 0.964) and control (r = 0.961) groups. The rates of RBC Na(+)-Li+ countertransport measured by NMR were significantly higher for hypertensive patients than for normotensive controls. The fact that the NMR method does not require cell membrane lysis, and its potential to reveal structural and mechanistic information on Li+ binding and transport in cellular systems, makes it promising for understanding the basis of Li+ transport variations in RBCs, and possibly other tissues, from hypertensive patients.