Abstract THE quantitative requirement for magnesium by laying hens has not been established. The National Research Council in 1966 listed the growing chicken's requirement as 500 mg. magnesium per kilogram of feed, but gave no estimate of the hen's requirement while Edwards et al. (1961) showed that the phosphorus level of the hen's diet had a marked influence on magnesium metabolism in laying hens. Their studies showed that high levels of phosphorus in the diet decreased 28Mg absorption from the small intestine and stimulated secretion of endogenous 28Mg from the hen's body into the gastrointestinal tract. Cox and Sell (1967) reported that magnesium deficiency in the laying hen caused a decrease in egg production, hypomagnesia, reduced feed intake and decreased egg size. High calcium levels in the diet appeared to hasten the development of magnesium deficiency. The studies reported in this paper were conducted to obtain information on the quantitative requirement . . .