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The plasma calcium and phosphate response to iv injection of PTH was studied in laying hens fed diets containing 2.26% or 5.00% calcium, both during periods of egg shell formation and when no egg was in the shell gland. Compared with results for mammals, the response was considerably larger, more rapid and more transient. With PTH injection at 0 min, plasma calcium (5.00% calcium diet only) and phosphate were significantly decreased at 5 min. This was followed by an increase in the concentration of both ions until a maximum was reached between 20 and 30 min. After 60 min, plasma levels of pullets fed the 2.26% calcium diet were again practically normal. The maximum plasma calcium and phosphate response for the 5.00% calcium diet was more than twice as large as for the 2.26% calcium diet, while the stage in the egg cycle had no effect on the response curves. During the period of hypercalcemia the increases in plasma calcium and phosphate were closely correlated (r = 0.733, p < 0.001), suggesting that the release of these two ions into blood was caused by a common mechanism. The rapid decay of the PTH response and its dependence on dietary calcium may explain the results of some investigators who found little or no response to parathyroid hormone in laying hens. (Endocrinology 92: 853, 1973) D URING the formation of each egg shell laying hens fed a diet containing 1.95% calcium mobilize about 4% of their skeletal calcium (1). Although it is likely that parathyroid hormone (PTH) plays a role in synchronizing egg formation and the concomitant changes in skeletal metabolism (2) surprisingly little is known about the effects of PTH in laying hens. One reason may be that a number of early investigators, cited by Polin et al. (3), were unable to detect any effect of PTH on blood calcium in birds. More recent studies have shown that these negative results were probably due to failure to examine calcium levels early enough (3, 4). It has been reported (4) that laying hens exhibit significant hypercalcemia 8 min after iv injection of 20 U/kg PTH and a similarly rapid response has been reported for chicks and Japanese quail (5).Information of the phosphatemic response