A deficiency in microsomal glucose-6-phosphatase (G6Pase) activity causes glycogen storage disease type 1 (GSD-1), a clinically and biochemically heterogeneous group of diseases. It has been suggested that catalysis by G6Pase involves multiple components, with defects in the G6Pase catalytic unit causing GSD-1a and defects in the putative substrate and product translocases causing GSD-1b, 1c, and 1d. However, this model is open to debate. To elucidate the G6Pase system, we have examinedG6PasemRNA expression, G6Pase activity, and glucose 6-phosphate (G6P) transport activity in the murine liver and kidney during normal development. In the liver,G6PasemRNA and enzymatic activity were detected at 18 days gestation and increased markedly at parturition, before leveling off to adult levels. In the kidney,G6PasemRNA and enzymatic activity appeared at 19 days gestation and peaked at weaning, suggesting that kidney G6Pase may have a different metabolic role.In situhybridization analysis demonstrated that, in addition to the liver and kidney, the intestine expressedG6Pase.Despite the expression ofG6Pasein the embryonic liver, microsomal G6P transport activity was not detectable until birth, peaking at about age 4 weeks. Our study strongly supports the multicomponent model for the G6Pase system.