We have studied 250 human liver biopsy samples to determine the ontogeny of the microsomal glucose-6-phosphatase (EC 3.1.3.9) system. Human hepatic glucose-6-phosphatase enzyme activity develops at 11 weeks' gestation and slowly increases to approximately 10% of adult activity at term. In the first week after birth, activity rises to adult values. Increases in enzyme activity coincide with increasing concentrations of the glucose-6-phosphatase enzyme protein. The phosphate/pyrophosphate transport protein (T2) of the human hepatic glucose-6-phosphatase complex develops at a different rate from that of the enzyme. Our study shows that the development of rat and human glucose-6-phosphatase activities are completely different. We conclude that deficiencies of the proteins in the microsomal glucose-6-phosphatase complex can be diagnosed with much more certainty perinatally than prenatally.