Dietary phosphate (Pi) regulates the production of 1,25-dihydroxyvitamin D3. This control is blunted in hypophysectomized (HPX) rats but can be restored by growth hormone (GH) or IGF-I. The regulation of the vitamin D catabolism by Pi, GH and IGF-I is less clear. In the present study, we found that the activity and transcript levels of the catabolic enzyme, vitamin D-24-hydroxylase (24-OHase) were decreased 3- and 5-fold, respectively, during Pi restriction in normal rats, but this effect is greatly reduced in HPX rats. Examination of the serum chemistries revealed that HPX rats on the high Pi diet had lower serum Pi levels than normal rats on this diet, presumably due to the known defective reabsorption of Pi by HPX rats. Treatment of HPX rats, adapted to a 0.6% P diet, with GH (150 micrograms) or IGF-I (80 micrograms) suppressed 24-OHase mRNA levels by 88% and 64%, respectively, by 20 hours and these effects were preceded by decreases in serum Pi. Our findings show that the 24-OHase is regulated by dietary Pi and this control is modulated by hypophysectomy, GH and IGF-I.