Six patients with primary hyperparathyroidism were studied during the first seven days after the operative removal of the parathyroid adenoma with special emphasis on biochemical and hormonal changes during the first 24 h. Serum parathyroid hormone (PTH) levels fell abruptly after the parathyroidectomy and normalized within 3 h. The half-life of the biologically inert c-terminal PTH-fragment (M.W. 7000-7500) was calculated to be about 180 min. No significant changes in serum calcitonin levels were found. The serum phosphorus levels, which were already low pre-operatively, decreased transiently but significantly during the first 90 min after the removal of the parathyroid adenoma. This fall in serum phosphorus preceded a slow decrease of the calcaemia. During the first post-operative week the calcaemia continued to decline, while serum phosphorus levels increased. The pre-operative cholesterol levels were low compared to age-paired normal Belgians. During the first post-operative week the cholesterolaemia decreased even more, whereas at long term follow-up a clearcut increase of the serum cholesterol levels has to be expected.