One hundred and four hens of parential stock Astra S were kept in individual cages. Two eggs per hen were taken for analyses at 30, 38, 46, 54 and 62 weeks of hens' age. The total number of 845 eggs were tested. Shell surface area (SSA) was calculated according to the formuła: SSA = egg weight ^0.667 x 4.67, and shell density according to the ratio of weight of dried shell without membranes to SSA. Ca and Mg contents in dried shell without membranes were determined using atomie spectrophotometry absorption methods. Egg weight increased throughout the laying period, whereas egg shell weight only up to 46 weeks. After 46 weeks shell thickness declined significantly (P<0.01). The highest breakage strength was found at 38 weeks, however, the highest shell density was at 46 and 54 weeks. The highest levels of Ca and Mg in 1 g of shell and their total contents in the shell were at 38 weeks. The Mg:Ca ratio reached the maximum values at 54 weeks. During the laying period not only the total amount of Ca and Mg in shells was undergoing changes but also the chemical composition of shells. These traits proved to be strictly related to the egg shell breakage strength.