It is known that 1,25-dihydroxycholecalciferol [1,25(OH)2D3] mobilizes inorganic phosphate (Pi) from gut and bone, thus increasing the entry of Pi into the extracellular space (ECS). In this work, we have investigated whether 1,25(OH)2D3 could also facilitate the net exit of Pi out of this pool into certain soft tissue and/or bone compartments in thyroparathyroidectomized (TPTX) rats. Sham-operated, TPTX, and TPTX rats treated with 1,25(OH)2D3 (26 pmol/day ip for 1 wk) were infused with a known amount of Pi over a given time. The amount of Pi excreted in the urine in excess of base-line values and the rise of the phosphatemia ([Pi]p) were measured. The difference between these two parameters corresponded to the net retention of infused Pi in the whole animal (RAPi) for a given rise in [Pi]p. The results show that RAPi was markedly diminished in TPTX as compared with sham-operated animals. This decrease could be corrected by 1,25(OH)2D3 treatment. Chronic reduction in the level of calcemia in TPTX 1,25(OH)2D3-supplemented rats as induced by decreasing dietary Ca intake also diminished RAPi. These effects could not be ascribed to the known actions of the experimental maneuvers on the renal excretion of Pi. Since infusions were chosen to obtain similar rises in [Pi]p and started from same levels of [Pi]p in animals of the same weight, the differences in RAPi were not due to variation of Pi retention in the ECS.(ABSTRACT TRUNCATED AT 250 WORDS)