Although α‐glycerolphosphate dehydrogenase activity in slow growing mouse mammary tumors is only 5% of that in normal mammary glands of lactating mice, the cellular concentration of α‐glycerolphosphate in both tissues is similar and is ca. 12 times that of dihydroxyacetone phosphate. Thus a positive correlation between the activity of glycerolphosphate dehydrogenase and the intracellular concentration of glyceride‐glycerol precursors could not be demonstrated.