Although calcitonin is a natural, nontoxic, and rapid inhibitor of bone resorption, its use in the treatment of hypercalcemia is limited because of its transient efficacy and the need for repeated parenteral administration. In normal subjects, suppositories of calcitonin have been shown to be biologically active. Peak plasma concentrations of salmon calcitonin after rectal administration in six normal subjects were similar to those measured after parenteral administration. To evaluate the efficacy of calcitonin suppositories in disease states, 10 patients with moderate hypercalcemia due to malignancy were treated with salmon calcitonin, administered as suppositories containing 300 MRC units, three times a day for seven days. The mean plasma calcium level decreased significantly from 2.96 +/- 0.09 mmol/liter to 2.57 +/- 0.09 after one week (p less than 0.005) and rose again after discontinuation of treatment to 2.86 +/- 0.09 mmol/liter one week later. Urinary calcium and hydroxyproline values decreased during treatment and rose after discontinuation of treatment. The plasma calcium level decreased rapidly in six patients, becoming normal in five; three patients showed only a partial response, and one patient had no response at all. No side effects were observed, and clinical improvement was noted in nine of the 10 patients. Little or no response was observed in patients with extremely high urinary calcium or, to a lesser extent, high hydroxyproline excretion. There was a significant negative correlation between the maximal decrease in plasma calcium concentration and initial urinary calcium excretion (r -0.78, p less than 0.01). Thus, salmon calcitonin administered by the rectal route appears to be an easy, safe, and effective treatment of moderate hypercalcemia without apparent side effects.