Lithium concentrations in saliva, plasma and red blood cells were measured in: 1) six hospitalized patients under long-term lithium therapy, at 2, 5, 9 and 24 hours after oral doses of 24 mEq Li acetate and 2 or 12 hours after 8 mEq Li acetate; and 2) 10 outpatients under chronic lithium treatment at two occasions 8 days apart. With changing plasma concentrations, [Li] saliva varied without any notable time lag. [Li] saliva was always much higher than [Li] plasma. The ratio [Li] saliva/ [Li] plasma water averaged 3.2 +/- 0.2 in 62 determinations, but varied widely at different times after oral lithium in the same individuals and less widely between different individuals. "Prediction" of plasma lithium concentration from measured [Li] saliva appears hazardous, and may provide reliable indications only if [Li] saliva is measured repeatedly. Salivary lithium concentrations were not correlated with either potassium or sodium concentrations. Lithium concentrations in red blood cells were always lower than in plasma: [Li] red blood cell water/ [Li] plasma water averaged 0.37 +/- 0.03. With changing plasma concentrations, rise and fall of red blood cell lithium lagged considerably behind plasma changes. This resulted in a rise of the red blood cell/plasma concentration ratio from a very low value 2 hours after an oral dose to a rather high value 24 hours after an oral dose.