The pharmacokinetics of lithium uptake was measured by 7Li NMR spectroscopy at 24.83 MHz in vivo in the brain and muscle of a normal subject and a patient suffering from bipolar affective disorder, using a modified General Electric Signa clinical magnetic resonance imaging system. Comparison was made to standard phantoms to estimate Li concentrations. The levels of Li in brain and muscle were similar, were typically less than the level in serum, and generally tracked the level in serum. The Li level at steady state in the brain of a patient suffering from schizoaffective disorder was measured over a 7-month period. Substantial variation was seen, which showed some correlation with serum level. Serum level peaked about 2 h after a single 300-mg dose at steady state, and muscle level, immediately thereafter. Brain level peaked considerably later at 4 h. Localized in vivo7Li NMR spectroscopy was demonstrated by acquisition of a 125-cm3 DRESS slice from the occipital region in less than 7 min. © 1990 Academic Press, Inc.