The development of the enzyme Na,K-ATPase was studied in hepatic tissue from Sprague-Dawley rats at 18-20 days of gestation, 1, 3, 5, 7, 14, 21, and 30 days, and young adulthood (45 days). A developmental pattern was demonstrated for total and membrane-associated Na,K-ATPase activity. The activity in tissue homogenate, expressed per gram tissue, increased from late fetal life, 43.4 +/- 1.3 mumol Pi g liver-1 h-1, until 21 days of age, when an adult level of 187.3 +/- 19.6 mumol Pi g liver-1 h-1 was attained. A less pronounced ontogenic pattern was observed when enzyme levels were expressed as specific activity per milligram protein. The activity profile in a crude membrane preparation was similar. The potential for stimulation of enzyme activity by glucocorticoids was studied in 11-day-old animals injected with cortisone acetate (10 mg 100 g body weight-1) for 3 days. Enzyme specific activity was inducible: Specific activity was greater in cortisone-treated animals, 1.794 +/- .043 mumol Pi mg protein-1 h-1, versus controls, 1.258 +/- 0.043 mumol Pi mg protein-1 h-1 (p less than 0.01). We postulate that this developmental pattern for hepatic Na,K-ATPase activity may be a reflection of, or a contributing factor in, the ontogeny of sodium-dependent hepatic transport, such as that for bile salts.