Magnesium metabolism has been studied in Hyalophora cecropia during the larval-pupal transformation (LPT) (Jungreis, A. M., Am. J. Physiol. 224: 27--30, 1973). Throughout this period, it accumulated in midgut (0.5 M at ecdysis), presumably as osmotically inactive Mg3(PO4)2. Glycerol accumulation in hemolymph was also first noted during this period. Since total alpha-glycerol phosphate present in hemolymph declined between the larval and pupal stages of development, the relationship between magnesium and alpha-glycerol phosphate metabolism was studied. Specific and total alpha-glycerol phosphatase (degradative) and glycerol kinase (synthetic) enzyme activities were measured in fat-body and midgut tissue throughout the LPT. At both feeding larval and diapause pupal stages in development in both tissues, total glycerol kinase activity is greater than that of alpha-glycerol phosphatase with degradative/synthetic activity ratios of 0.2--0.5. In fat body, ratios remained constant or shifted in the direction of synthesis during the LPT, whereas those measured in midgut tissue increased in the direction of degradation with a maximum ratio of 5.8 noted following spinning. The increase in degradative/synthetic activity ratios in midgut tissue is attributed to a greater rate of loss of glycerol kinase than alpha-glycerol phosphatase enzyme activity. Orthophosphate, presumably released from alpha-glycerol phosphate within the cells of the pharate pupal midgut tissue, combines with magnesium to form osmotically inactive Mg3(PO4)2 crystals.