1. The effect of calcitonin, a large amount of calcium given orally, pentagastrin and glucagon on plasma 47Ca radioactivity curves in subjects pretreated with 47Ca was examined.
2. A sudden increase of plasma radioactivity after intravenous administration of calcitonin and pentagastrin and after the oral calcium load was observed in normal subjects; the intravenous infusion of glucagon was less effective.
3. Two thyroparathyroidectomized patients who responded to the calcitonin infusion did not respond to the oral calcium load.
4. These data may be considered to offer indirect evidence of endogenous calcitonin secretion in man.