Adenine nucleotides have been measured in fetal and newborn kidneys of rat using the luciferine-luciferase method. In fetuses, between days 18 and 21 of gestation there is a drop of the relative amount of ATP in the renal nucleotide pool. Consequently, the kidneys of 21 days-old fetuses have lowered ATP/ADP ratio (3.6) and energy charge (0.80) compared with values found on day 18 (6.9 and 0.91, respectively); this relative energy deficit is heightened in progesterone induced postmaturity. One hour after delivery whether the gestational stage is 21, 22 or 23 days, there is a rise in ATP and a decrease in AMP content which restore a high energy level in kidney of the newborn and a 30% increase in the total adenine nucleotide pool.