Renal clearance studies were performed in chicks 1, 5, and 9 days after hatching. Calcium gluconate was infused to block endogenous parathyroid hormone (PTH) secretion, whereas ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA) was infused to stimulate endogenous PTH secretion. PTH, dibutyryl adenosine 3',5'-cyclic monophosphate (DBcAMP), or vehicle was administered intravenously. In 9-day-old birds, urinary cAMP and inorganic phosphate excretion fell dramatically after calcium loading and rose significantly after PTH, DBcAMP, or EGTA administration. These manipulations had no significant effect on excretion of calcium or inorganic phosphate in 1-day-old hatchlings. Five day-old-chicks gave an intermediate response. All three age groups, as well as 15-day-embryos, showed sharp increases in urinary cAMP values after PTH administration. Thus the adult response pattern to PTH appears to develop gradually during the first week after hatching or to be suppressed during the perinatal period, whereas the second-messenger response to the hormone, indicating hormone-receptor interaction, is present from late embryogenesis onward.