Two experiments were carried out to investigate the Ca and P metabolism of hens laying thick- (THK) or thin-shelled (THN) eggs on shell-forming days (SF) and days on which shell formation does not occur (NSF). In Experiment 1, 10 hens identified as producing THK or 10 hens producing THN eggs were offered a diet containing 3.5% Ca and .4% available P. Feed consumption, egg production, egg weight, egg deformation, and shell weight were recorded over 8 days. Shells and excreta were collected daily for Ca and P analysis. Feed, Ca, and P intake did not differ significantly (P greater than 0.5) between the two groups of hens, however, feed intake and Ca retention increased significantly (P less than .05) on SF compared with NSF days. The THK hens retained significantly (P less than .05) more Ca compared with THN hens. No differences (P greater than .05) were recorded for egg production, although there were differences (P less than .05) in egg weight and shell deformation between the two groups of hens. Increased egg weight did not account for differences (P less than .05) in eggshell deformation. Although percentage shell Ca was not significantly different (P greater than .05), total shell Ca was different (P less than .05) between the two groups. In Experiment 2, both groups of birds (THK and THN) were allowed simultaneous access to two different diets, a high-energy, high-protein diet low in Ca and P, and a low-energy, low-protein diet high in Ca and P.(ABSTRACT TRUNCATED AT 250 WORDS)