The effects of the intravenous administration of atropine or magnesium on pancreatic secretion which has been stimulated by secretin and induced hypercalcaemia have been studied in man. In the presence of secretin (0.5 CU/kg.h) the infusion of Ca2+ (0.3 mmol/kg.105 min) resulted in an increase in secretion of enzymes by 100-200%, and in that of Ca2+ and Mg2+ by 50-100% without affecting fluid and bicarbonate secretion. The additional injection of atropine (0.5 mg i.v. and 0.5 mg s.c.) were followed by a prompt fall in enzymes but not in Ca2+ and Mg2+ to the secretin-stimulated values. The additional infusion of Mg2+ (0.12 mmol/kg.45 min) to the Ca2+-infusion did not alter the secretion of enzymes, Ca2+ or Mg2+ compared with the calcium infusion alone. It is suggested that the hypercalcaemic stimulus depends on an intact innervation of the acinar cells. In these experiments the secretion of Ca2+ and Mg2+ seem to originate mainly from extracellular fluxes.