The authors investigated the effect of acute hypercalcaemia induced by a 2-hour intravenous infusion of calcium gluconate (8.9 mg Ca2+/kg b. w.) on the lactotrophic secretory reserve assessed by the test with insulin hypoglycaemia (delta PRL) and the effect of an intravenous bolus of 50 IU synthetic salmon calcitonin on the lactotrophic secretory reserve assessed by means of the TRH test (delta PRL). Acute hypercalcaemia inhibits PRL levels stimulated by insulin hypoglycaemia (p less than 0.01) as well as delta PRL (p less than 0.01). Calcitonin reduces PRL levels at rest and TRH stimulated levels (p less than 0.05 and p less than 0.01, respectively) as well as delta PRL (p less than 0.01). The prolactin inhibiting effect of calcitonin resembles markedly the effect of hypercalcaemia. The exact mechanism of these changes and the physiological impact of calcitonin on the regulation of PRL secretion is not known.