Erythrocyte ouabain maximum binding (V m a x level) represents the number of molecules of Na-K ATPase on the erythrocyte membrane. Erythrocyte V m a x levels and sodium potassium concentrations were studied in 38 mature and 57 premature infants, including 40 very low birth weight infants, 41 children and 16 adults, in order to clarify the development of erythrocyte Na-K ATPase related to mineral balance and thermogenesis. V m a x levels were the highest (22.9 +/- 8.9 nmol/l cells) for premature infants; correlated negatively with gestational age (r = -0.77, P < 0.01). V m a x levels correlated positively with the ratio of erythrocyte potassium to sodium concentration (IC-K/Na) at <20 nmol/l cells of V m a x levels (r = 0.72, P < 0.01), but not at \>=20 nmol/l cells (r = 0.23). V m a x levels in very low birth weight infants, especially <28 weeks postconception, decreased gradually with postnatal days. Ln(V m a x ) levels correlated negatively with postconceptional age (r = -0.75, P < 0.01). Under 28 weeks postconception, V m a x levels were high, but not correlated with IC-K/Na. It is probable that individual Na-K ATPase molecules may be less active, and the large numbers of Na-K ATPase molecules may compensate for the individual lower activity.