The effect of moderate hyperglycemia on renal ATP production and ATPase activity of rat fetus was investigated using the experimental procedure of maternal continuous infusion of glucose during the last 5 days of gestation. Glucose-infused mothers and their fetuses showed a high level of glycemia (8.8 and 5.5 mM, respectively) and a high level of insulinemia (3 times higher than in controls). No change in either ATP or ADP concentration was detectable but an increase in NaK ATPase activity occurred without any change in Mg ATPase activity. These modifications should be the result of an enhanced Na/glucose cotransport leading to an enhanced extrusion of Na at the basolateral membrane. These results indicate that immature kidney is able to increase NaK ATPase activity to maintain Na homeostasis.