Plasma total calcium to magnesium ratios were observed as an index of muscular activity during pipping and hatching of chick and poult embryos. No significant changes were observed in plasma calcium (Ca) or magnesium (Mg) concentrations, but the total Ca to Mg ratio increased significantly just prior to pipping. Lighting the incubator during the last 5 days of incubation in significantly increased plasma Ca and Ca:Mg ratios. Lighting also resulted in significantly better hatchability and less late embryonic mortality. An increased plasma Ca:Mg ratio may be a physiological mechanism to provide increased muscular activity for the embryo in breaking the shell and emerging from it.