The site of outflux of 45Ca along the nephron were investigated using microinfusion technique in acutely thyroparathyroidectomized (TPTX), intact and TPTX Wistar rats substituted with parathyroid hormone (PTH). In all three groups 45Ca outflux occurred along the proximal tubule, the loop of Henle and along the distal tubule. After microinfusion into late distal tubules 45Ca recovery in ipsilateral urine was essentially complete for the TPTX group but was only 83 and 65% for the intact and the PTH substituted animals. Increases in microinfusion flow rate from 2--20 nl/min into early and middle distal tubules resulted in increased urinary recovery of 45Ca for all three groups. Similarily, increases in microinfusate Ca concentration from 0.2--2.0 and 5.0 mmol/l resulted in increased fractional urinary recovery of 45Ca both in the presence and absence of PTH. When 45Ca and 3H insulin containing solutions were continuously microinfused into early distal tubules for one hour periods an anticalciuric effect of PTH could be demonstrated. It is concluded that Ca, in addition to its outflux in the proximal tubule and in the loop of Henle, is reabsorbed in the distal tubule accessible to micropuncture. PTH acts anticalciuric at this latter site.