Previous studies from this laboratory have demonstrated a phosphaturic response to hydrocortisone. The present micropuncture studies were undertaken to localize this glucocorticoid-induced effect along the nephron. Adult female Sprague-Dawley rats were used and parathyroidectomized 3 h before the micropuncture. The animals were infused with modified Ringer solution (either without or with phosphate). Hydrocortisone increased both the end-proximal transtubular concentration gradient of inorganic phosphate, (TF/P)Pi, and the fractional delivery of Pi at the end of the proximal convolution significantly. The absolute reabsorption of Pi decreased from 95.1 +/- 29.5 (mean +/- SD) to 59.7 +/- 28.2 pmol/min (p less than 0.005) and the absolute reabsorption of Pi/single nephron glomerular filtration rate from 2.34 +/- 0.59 to 1.35 +/- 0.54 pmol/nl (p less than 0.001) during Pi-Ringer infusion. It is concluded that the glucocorticoid-induced phosphaturia is primarily a result of the inhibition of the reabsorption of Pi in the proximal tubule.