The precision of PTH measurement and the location of the implant allow the functioning of the transplanted tissue to be easily checked by separately measuring the PTH concentration in the arm bearing the transplant in comparison with the other side. In 10 of the 13 patients operated on by us it was observed that the transplant had grown and was functioning satisfactorily. In 2 further patients, in whom the PTH concentration alone gave no clear information on the functioning of the transplanted tissue, the tolerance test was for this purpose decisive. In fact the transplanted parathyroid tissue showed a rapid and clear response to pharmacodynamic stimulus, which demonstrated the viability of the transplant. We therefore venture to say that pharmacodynamic investigation is of a certain use in postoperative examinations of these patients, especially in doubtful cases. Unfortunately we have had no opportunity to perform a histological investigation of the implanted endocrine tissue, but this has already been reported on in print.