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Hy-Line W36® hens, 56-wk-old, were used to study the response to changing dietary Ca and P levels of hens laying eggs with heavy (HSW) or light shell weight (LSW). Three diets were fed: control diet [3.9% Ca and .55% total P (P t )], low-Ca diet (2.2% Ca and .55% P t ), and high-P diet (3.9% Ca and .90% P t ). In Experiment 2, one half of hens previously fed the low-Ca diet were changed to a Ca-deficient diet (1.7% Ca and .55% P t ), and the others continued to receive the diet previously fed. Also, one half of the hens fed the high-P diet were changed to a high-Ca, high-P diet (4.4% Ca and .90% P t ), and the others continued to receive the high-P diet. The control hens continued to receive the control diet. Shell weight was determined, and feed intake and egg production (EP) were recorded. Calcium and P intake, and Ca utilization were calculated. Plasma Ca and P, tibia breaking strength, and bone ash were determined.In both experiments egg production and shell weight were significantly lower for hens consuming both low-Ca and Ca-deficient diets than hens fed the control diet. The adverse effect was greater for HSW hens than for LSW hens. Many of HSW hens ceased production when fed the Ca-deficient diet. Calcium utilization was greater for HSW than for LSW hens. Increasing P in the diet from .55 to .90% P t had no significant effect on SW or EP. Increasing Ca in the diet from 3.9 to 4.4% and P from .55 to .90% P, had no significant effect on either EP, SW, or feed intake. Bone breaking strength and bone ash were not affected by dietary treatments. Plasma Ca and P concentrations were higher in the LSW than HSW hens. (