Recent data have shown that administration of prostaglandin inhibitors to patients with hypercalciuric nephrolithiasis decreased urinary calcium excretion, implying a possible role for prostaglandins in calcium excretion. To explore this hypothesis, we investigated the effect of single dose or 7 days' administrations of aspirin (100 mg/kg orally) or indomethacin (20 mg/kg, orally) on the urinary and serum concentrations of calcium, magnesium and inorganic phosphate. Experiments were performed in normocalcaemic and hypercalcaemic rats. Hypercalcaemia and hypercalciuria were induced in male Wistar albino rats by administration of vitamin D3 (20,000 IU/daily) for 7 days. Aspirin and indomethacin both significantly lowered the urinary calcium excretion in normocalciuric and hypercalciuric rats. The acute administration of indomethacin caused greater reduction of calcium excretion than that produced by the acute administration of aspirin, whereas aspirin showed greater activity than indomethacin after the long-term use of each. Aspirin induced hypocalcaemia in normocalcaemic rats and abolished the hypercalcaemia in hypercalcaemic rats. On the contrary, indomethacin, a specific prostaglandin biosynthesis inhibitor, increased serum levels of calcium. Hypophosphataemia was observed only after the administration of a single dose of aspirin in normocalcaemic rats, while the reduction of urinary phosphate excretion was investigated in hypercalciuric rats after the acute and chronic administration of indomethacin. Serum levels of phosphate were not altered significantly by acute or chronic administration of indomethacin. A single dose of indomethacin significantly reduced urinary excretion of magnesium in both groups of rats. However, the acute and chronic administration of aspirin resulted in non-significant changes in serum and urinary concentrations of magnesium. These data suggest that aspirin has hypocalcaemic and hypocalciuric actions while indomethacin has only a hypocalciuric effect. Aspirin may produce these actions by two mechanisms, one of them like that of indomethacin which is dependent on the inhibition of biosynthesis of prostaglandins, and another possible mechanism that is not related to the inhibition of prostaglandin biosynthesis. This suggestion may be supported by the discrepancy between the effects of aspirin and indomethacin on the renal handling and serum concentrations of magnesium and inorganic phosphate.