AbstractUptake of orthophosphate (Pi) by osteoblast‐like cells is known to be stimulated by parathyroid hormone (PTH), but effects on intracellular [Pi] have not been investigated. Here we show in rat osteoblast‐like cells (UMR 106‐06) that PTH (10−11to 10−7M) increases both32Pi uptake and cellular [Pi] by up to 50 per cent. 1,25 Dihydroxyvitamin D3(1,25D) (10−12to 10−6M) and salmon calcitonin (CT) (10−12to 10−6g ml−1) also increased cellular [Pi] (by up to 60 per cent), but the percentage increases in total cellular32Pi uptake were smaller. The effects of 1,25D were transient (observable at 80 min and 6 h but not 24 h), and were also observed with 24,25 dihydroxy‐ and 25 hydroxyvitamin D3. Transient degradation of organic phosphorus pools to Pi might contribute to this increased [Pi]. These pools remain to be identified but were not shown to be phospholipids. Foetal bovine serum also affected cellular [Pi]. Care is therefore needed in distinguishing direct hormonal effects on cellular [Pi] from indirect effects arising from changes in th