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Renal phosphorus handling was evaluated in 12 lithium carbonate-treated psychiatric patients. Serum phosphorus was normal and serum lithium values were within the therapeutic range in all subjects. Serum calcium concentrations measured in 6 of the patients were found to be within the normal range; in the same patients serum parathyroid hormone levels were normal in 4 and slightly elevated in 2. Phosphorus clearance (14 +/- 3 [se] ml/min) and tubular reabsorption of phosphorus (88 +/- 2%) during oral sodium bicarbonate loading were not significantly different from those in 10 healthy control subjects. In a subgroup of 5 patients and 5 control subjects, phosphorus excretion did not increase after bicarbonate loading. In these subjects, phosphorus excretion rates after bicarbonate loading were not different. Although experimental studies suggest that lithium inhibits renal cortical adenylate cyclase stimulation by parathyroid hormone, our data did not indicate any striking effect of long-term lithium administration on serum calcium and serum phosphorus or on renal phosphorus handling.