Plasma immunoreactive parathyroid hormone (iPTH), 1,25(OH)2D3, calcium and phosphate and urinary creatinine, calcium and phosphate were measured before and following unilateral nephrectomy in six kidney donors. Unexpectedly, plasma calcium rose, from 2.27 +/- 0.02 mmol/l (mean +/- SEM) to 2.41 +/- 0.03 mmol/l on day 7 and to 2.37 +/- 0.02 mmol/l on day 30 (P less than 0.02). A parallel rise in iPTH occurred, from 0.61 +/- 0.16 ng/ml initially, to 1.83 +/- 0.54 ng/ml on day 7 (P less than 0.05) and to 1.18 +/- 0.18 on day 30 (P less than 0.01). The ratio of maximal tubular reabsorption of phosphate to GFR (TmP/GFR) fell by day 2 (P less than 0.01), remaining reduced on day 30 (P less than 0.05). The significance of elevated iPTH in renal insufficiency was further assessed by determining the time course of the disappearance of iPTH after parathyroidectomy in three haemodialysis subjects. Fifty per cent baseline iPTH level occurred after an average of 104.7 min, suggesting that the assay did not predominantly recognize C-terminal PTH fragments. By day 2, plasma 1,25(OH)2D3 had fallen from 34.3 +/- 4.5 pg/ml to 22.8 +/- 3.8 pg/ml (P less than 0.001), but by day 4 had regained its pre-nephrectomy value. Our results suggest that hypocalcaemia may not be the sole stimulus to parathyroid hormone secretion. It is speculated that reduction in circulating 1,25(OH)2D3 may be involved.