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Experiments were performed to evaluate the effects of dietary available phosphorus (aP) and PTH infusion rates on avian urinary inorganic phosphate (Pi) excretion. In experiment I, female domestic fowl were fed diets containing low (0-45%) or high (0·83%) aP for 2–4 weeks prior to renal function studies. Pi excretion was significantly higher for birds fed the high-aP diet than for birds fed the low-aP diet. PTH was infused (60–240 units kg body mass−1 h−1) unilaterally into the renal portal system. Para-aminohippuric acid (PAH), included in the unilateral infusate as a marker for effective renal portal perfusion, indicated that PTH must have been delivered to the peritubular surfaces of the infused kidney. However, bilateral but not unilateral phosphaturia occurred, and there were no significant differences in the phosphaturic responses to PTH when low-and high-aP diet treatment groups were compared. In experiment II, PTH was infused at rates of l–5 units h−1. Infusing PTH at 5 units h−1 caused a unilateral increase in urine flow but the phosphaturic response was still bilateral. It appears unlikely that unilateral renal portal PTH infusions can be used to trigger unilateral phosphaturia in domestic fowl.