OBJECTIVE: This study examined the interaction between lithium and diuretics, comparing both the pharmacokinetic and the pharmacodynamic variable of hydrochlorothiazide, furosemide, and placebo. METHOD: The study, which took place in an outpatient research clinic of a university hospital, used a double-blind, placebo-controlled crossover design. The subjects were normal, healthy male volunteers who responded to recruitment announcements. Thirteen subjects entered and completed the study. All subjects took lithium, 300 mg b.i.d., for 6 weeks. Hydrochlorothiazide, 25 mg b.i.d.; furosemide, 20 mg b.i.d.; and placebo were given during weeks 2, 4, and 6 in a random order of assignment. Serum lithium levels and indices of diuretic activity were measured during each week. RESULTS: The subjects' serum lithium levels after 5 days of taking hydrochlorothiazide were significantly higher than after 5 days of taking furosemide and placebo. At the doses studied, hydrochlorothiazide was also more potent than furosemide in increasing plasma renin activity, increasing sodium excretion, and decreasing lithium excretion. CONCLUSIONS: The observed differences between diuretics in effects on serum lithium may have been due to differences in the potency of the diuretics at the doses studied as well as in the site of action of the diuretic effect. The results must be interpreted cautiously, however, because the effects were small and of questionable clinical significance, and the study used healthy volunteers and low doses of lithium instead of psychiatric patients and the usual therapeutic levels of lithium.Abstract Teaser