Background/Aims: Phosphorus reabsorption in the kidney is regulated by parathyroid action on the sodium phosphorus cotransporter (Na-Pi cotransporter). There is some evidence that estrogen may also regulate renal phosphorus handling but it is not known if this is an effect of estrogen on filtered phosphorus load. Methods: This study examined the effect of estrogen on renal phosphorus handling during infusion with calcium or EDTA. Six month old Sprague Dawley rats were bilaterally oophorectomized (OOX) or underwent a sham operation under ether anaesthesia 6 weeks before undergoing infusion with calcium chloride or EDTA. A second study examined renal phosphorus handling after estrogen replacement in the COX rat injected with 20 mug estradiol valerate, or vehicle alone, prior to calcium infusion. Results: A comparison of filtered phosphorus load with renal phosphorus excretion indicated that the estrogen replete rat had a higher renal excretion of phosphorus when infused with both calcium (p = 0.004) and EDTA (p = 0.037) at all filtered phosphorus loads. A similar analysis in calcium infused, estrogen replaced, COX rats indicated an effect of estrogen to increase renal phosphorus excretion (p = 0.007) at all filtered phosphorus loads. COX resulted in a mild metabolic alkalosis, an effect not reversed by estrogen administration, that was not related to renal phosphorus excretion. OOX decreased renal sodium excretion and uncoupled the relation between renal phosphorus and sodium excretion, an effect reversed by estrogen replacement. Conclusion: Ovarian hormone deficiency in the rat results in a decrease in renal phosphorus excretion. This finding is comparable to effects of the menopause and hormone replacement therapy in postmenopausal women. Copyright (C) 2001 S. Karger AG, Basel.